networking: fix unexpected negative or zero readlen

To avoid copying buffers to create a large Redis Object which
exceeding PROTO_IOBUF_LEN 32KB, we just read the remaining data
we need, which may less than PROTO_IOBUF_LEN. But the remaining
len may be zero, if the bulklen+2 equals sdslen(c->querybuf),
in client pause context.

For example:

Time1:

python
>>> import os, socket
>>> server="127.0.0.1"
>>> port=6379
>>> data1="*3\r\n$3\r\nset\r\n$1\r\na\r\n$33000\r\n"
>>> data2="".join("x" for _ in range(33000)) + "\r\n"
>>> data3="\n\n"
>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> s.settimeout(10)
>>> s.connect((server, port))
>>> s.send(data1)
28

Time2:

redis-cli client pause 10000

Time3:

>>> s.send(data2)
33002
>>> s.send(data3)
2
>>> s.send(data3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
socket.error: [Errno 104] Connection reset by peer

To fix that, we should check if remaining is greater than zero.

src/networking.c

@@ -1439,7 +1439,7 @@ void readQueryFromClient(aeEventLoop *el, int fd, void *privdata, int mask) {
     {
         ssize_t remaining = (size_t)(c->bulklen+2)-sdslen(c->querybuf);
 
-        if (remaining < readlen) readlen = remaining;
+        if (remaining > 0 && remaining < readlen) readlen = remaining;
     }
 
     qblen = sdslen(c->querybuf);