networking: optimize parsing large bulk greater than 32k

If we are going to read a large object from network
try to make it likely that it will start at c->querybuf
boundary so that we can optimize object creation
avoiding a large copy of data.

But only when the data we have not parsed is less than
or equal to ll+2. If the data length is greater than
ll+2, trimming querybuf is just a waste of time, because
at this time the querybuf contains not only our bulk.

It's easy to reproduce the that:

Time1: call `client pause 10000` on slave.

Time2: redis-benchmark -t set -r 10000 -d 33000 -n 10000.

Then slave hung after 10 seconds.

src/networking.c

@@ -1294,19 +1294,22 @@ int processMultibulkBuffer(client *c) {
 
             c->qb_pos = newline-c->querybuf+2;
             if (ll >= PROTO_MBULK_BIG_ARG) {
-                size_t qblen;
-
                 /* If we are going to read a large object from network
                  * try to make it likely that it will start at c->querybuf
                  * boundary so that we can optimize object creation
-                 * avoiding a large copy of data. */
-                sdsrange(c->querybuf,c->qb_pos,-1);
-                c->qb_pos = 0;
-                qblen = sdslen(c->querybuf);
-                /* Hint the sds library about the amount of bytes this string is
-                 * going to contain. */
-                if (qblen < (size_t)ll+2)
-                    c->querybuf = sdsMakeRoomFor(c->querybuf,ll+2-qblen);
+                 * avoiding a large copy of data.
+                 *
+                 * But only when the data we have not parsed is less than
+                 * or equal to ll+2. If the data length is greater than
+                 * ll+2, trimming querybuf is just a waste of time, because
+                 * at this time the querybuf contains not only our bulk. */
+                if (sdslen(c->querybuf)-c->qb_pos <= (size_t)ll+2) {
+                    sdsrange(c->querybuf,c->qb_pos,-1);
+                    c->qb_pos = 0;
+                    /* Hint the sds library about the amount of bytes this string is
+                     * going to contain. */
+                    c->querybuf = sdsMakeRoomFor(c->querybuf,ll+2);
+                }
             }
             c->bulklen = ll;
         }