qapi: input_type_enum(): fix error message

The enum string is pointed to by 'enum_str' not 'name'. This bug
causes the error message to be:

{ "error": { "class": "InvalidParameter",
             "desc": "Invalid parameter 'null'",
             "data": { "name": "null" } } }

Signed-off-by: Luiz Capitulino <[email protected]>
Reviewed-by: Amos Kong <[email protected]>

qapi/qapi-visit-core.c

@@ -298,7 +298,7 @@ void input_type_enum(Visitor *v, int *obj, const char *strings[],
     }
 
     if (strings[value] == NULL) {
-        error_set(errp, QERR_INVALID_PARAMETER, name ? name : "null");
+        error_set(errp, QERR_INVALID_PARAMETER, enum_str);
         g_free(enum_str);
         return;
     }