mime: address scan-build warnings

mime: address scan-build warnings

util-decode-mime.c:189:31: warning: Use of memory after it is freed [unix.Malloc]
            lastSibling->next = entity->child;
            ~~~~~~~~~~~~~~~~~ ^
util-decode-mime.c:827:24: warning: Potential leak of memory pointed to by 'val' [unix.Malloc]
        state->hname = NULL;
                       ^~~~
/usr/lib/llvm-16/lib/clang/16/include/stddef.h:89:24: note: expanded from macro 'NULL'
 #  define NULL ((void*)0)
                       ^
2 warnings generated.

Improve error handling and add assert to avoid these warnings.

Bug: #3147.