## Description
The `DecCoins.MulDecTruncate` function is supposed to panic instead of returning `nil` when multiplier `d` is zero. This fixes that behavior and updates the related test `decCoinTestSuite.TestDecCoins_MulDecTruncate` to expect a panic when multiplier `d` is zero.
Closes: #9790
---
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evancholmberg
08db28711b