fix(math): Dec marshal shouldn't have side effects (#15506)

## Description

Ref: https://github.com/cosmos/cosmos-sdk/issues/15170
Included in: #15515 

Required to resolve a discrepancy between the sign mode being implemented in #15170 and the existing legacy one.

When calling tx builder [SetMsgs](https://github.com/cosmos/cosmos-sdk/blob/4a6a1e3cb8de459891cb0495052589673d14ef51/x/auth/tx/builder.go#L208) messages are coerced to an any type in [NewAnyWithValue](https://github.com/cosmos/cosmos-sdk/blob/4a6a1e3cb8de459891cb0495052589673d14ef51/codec/types/any.go#L61). This marshals a message as proto bytes. If a message has cosmos.Dec typed fields, and they are set to nil, the will be set to 0 during seriazliation in the [Dec.Marshal](https://github.com/cosmos/cosmos-sdk/blob/88909d6f2b86dee6969a76e1e0e3c106fe8fd20c/math/dec.go#L801) implementation.

This side effect *changes* the output of a JSON marshal before and after the call to Dec.Marshal, as shown below.

```go
rates := &stakingtypes.CommissionRates{}
MarshalJson(rates)
// {"rate":"0","max_rate":"0","max_change_rate":"0"}

MarshalProto(rates)

MarshalJson(rates)
//{"rate":"0.000000000000000000","max_rate":"0.000000000000000000","max_change_rate":"0.000000000000000000"}
```

An integration is test is also committed asserting the (now fixed) behavior above.  Its asserts should be changed once a release of math is made and `/tests/go.mod` is updated.



---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

* [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
* [ ] added `!` to the type prefix if API or client breaking change
* [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
* [ ] provided a link to the relevant issue or specification
* [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/building-modules)
* [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#testing)
* [ ] added a changelog entry to `CHANGELOG.md`
* [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
* [ ] updated the relevant documentation or specification
* [ ] reviewed "Files changed" and left comments if necessary
* [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

* [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
* [ ] confirmed `!` in the type prefix if API or client breaking change
* [ ] confirmed all author checklist items have been addressed 
* [ ] reviewed state machine logic
* [ ] reviewed API design and naming
* [ ] reviewed documentation is accurate
* [ ] reviewed tests and test coverage
* [ ] manually tested (if applicable)

math/CHANGELOG.md

@@ -34,6 +34,12 @@ Ref: https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.j
 
 # Changelog
 
+## [Unreleased] 
+
+### Bug Fixes
+
+* [#15506](https://github.com/cosmos/cosmos-sdk/issues/16605) fix: Dec marshal shouldn't have side effects
+
 ## [math/v1.0.0-rc.0](https://github.com/cosmos/cosmos-sdk/releases/tag/math/v1.0.0-rc.0) - 2023-03-13
 
 ### Features
@@ -43,6 +49,7 @@ Ref: https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.j
 ### Bug Fixes
 
 * [#14922](https://github.com/cosmos/cosmos-sdk/issues/14922) check for negative precision
+* [#15506](https://github.com/cosmos/cosmos-sdk/issues/16605) fix: Dec marshal shouldn't have side effects
 
 ### Testing
 

math/dec.go

@@ -799,19 +799,21 @@ func (d LegacyDec) MarshalYAML() (interface{}, error) {
 
 // Marshal implements the gogo proto custom type interface.
 func (d LegacyDec) Marshal() ([]byte, error) {
-	if d.i == nil {
-		d.i = new(big.Int)
+	i := d.i
+	if i == nil {
+		i = new(big.Int)
 	}
-	return d.i.MarshalText()
+	return i.MarshalText()
 }
 
 // MarshalTo implements the gogo proto custom type interface.
 func (d *LegacyDec) MarshalTo(data []byte) (n int, err error) {
-	if d.i == nil {
-		d.i = new(big.Int)
+	i := d.i
+	if i == nil {
+		i = new(big.Int)
 	}
 
-	if d.i.Cmp(zeroInt) == 0 {
+	if i.Cmp(zeroInt) == 0 {
 		copy(data, []byte{0x30})
 		return 1, nil
 	}