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.

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];