Do not crash on using _slot and _offset suffixes on their own

test/libsolidity/syntaxTests/inlineAssembly/storage_reference_empty_offset.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure {
+        assembly {
+            _offset
+        }
+    }
+}
+// ----
+// DeclarationError: (75-82): In variable names _slot and _offset can only be used as a suffix.

test/libsolidity/syntaxTests/inlineAssembly/storage_reference_empty_slot.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure {
+        assembly {
+            _slot
+        }
+    }
+}
+// ----
+// DeclarationError: (75-80): In variable names _slot and _offset can only be used as a suffix.