Fix null pointer dereference validating struct.new (#7688)

We would previously dereference a null descriptor operand to struct.new
when the allocated type has a descriptor. This only happens in invalid
IR, but the validator needs to handle that gracefully.

Author: tlively

src/wasm/wasm-validator.cpp

@@ -3163,6 +3163,12 @@ void FunctionValidator::visitStructNew(StructNew* curr) {
                   curr,
                   "struct.new of type without descriptor should lack one");
   } else {
+    if (!shouldBeTrue(
+          curr->desc,
+          curr,
+          "struct.new of type with descriptor requires descriptor operand")) {
+      return;
+    }
     shouldBeSubType(curr->desc->type,
                     Type(*descType, Nullable, Exact),
                     curr,