Modules: defrag CB should take robj, not sds (#13627)

Added a log of the keyname in the test modules to reproduce the problem
(tests crash without the fix)

tests/modules/defragtest.c

@@ -161,13 +161,14 @@ size_t FragFreeEffort(RedisModuleString *key, const void *value) {
 }
 
 int FragDefrag(RedisModuleDefragCtx *ctx, RedisModuleString *key, void **value) {
-    REDISMODULE_NOT_USED(key);
     unsigned long i = 0;
     int steps = 0;
 
     int dbid = RedisModule_GetDbIdFromDefragCtx(ctx);
     RedisModule_Assert(dbid != -1);
 
+    RedisModule_Log(NULL, "notice", "Defrag key: %s", RedisModule_StringPtrLen(key, NULL));
+
     /* Attempt to get cursor, validate it's what we're exepcting */
     if (RedisModule_DefragCursorGet(ctx, &i) == REDISMODULE_OK) {
         if (i > 0) datatype_resumes++;