Fix an hypothetical issue in processMultibulkBuffer().

src/networking.c

@@ -987,15 +987,19 @@ int processMultibulkBuffer(redisClient *c) {
 
             pos += newline-(c->querybuf+pos)+2;
             if (ll >= REDIS_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,pos,-1);
                 pos = 0;
+                qblen = sdslen(c->querybuf);
                 /* Hint the sds library about the amount of bytes this string is
                  * going to contain. */
-                c->querybuf = sdsMakeRoomFor(c->querybuf,ll+2-sdslen(c->querybuf));
+                if (qblen < ll+2)
+                    c->querybuf = sdsMakeRoomFor(c->querybuf,ll+2-qblen);
             }
             c->bulklen = ll;
         }