Assert that setDeferredAggregateLen isn't called with negative value

In case the redis is about to return broken reply we want to crash
with assert so that we are notified about the bug. see #7687.

(cherry picked from commit 8b428cf0f7ce5489fbdf4640a1dd36357f3dc5f0)

src/networking.c

@@ -492,6 +492,7 @@ void *addReplyDeferredLen(client *c) {
 
 /* Populate the length object and try gluing it to the next chunk. */
 void setDeferredAggregateLen(client *c, void *node, long length, char prefix) {
+    serverAssert(length >= 0);
     listNode *ln = (listNode*)node;
     clientReplyBlock *next;
     char lenstr[128];