util/mime: fix memory leak

Fix memory leak at util-decode-mime:MimeDecInitParser, which
root cause is not-freeing allocated memory for mimeMsg

Bug: #6745
(cherry picked from commit 231c892bef)

src/util-decode-mime.c

@@ -2469,6 +2469,7 @@ MimeDecParseState * MimeDecInitParser(void *data,
     PushStack(state->stack);
     if (state->stack->top == NULL) {
         SCFree(state->stack);
+        SCFree(state->msg);
         SCFree(state);
         return NULL;
     }