refine buffer read socket logic.

@@ -62,10 +62,10 @@ int SrsBuffer::ensure_buffer_bytes(SrsSocket* skt, int required_size)
 {
 	int ret = ERROR_SUCCESS;
 
-	ssize_t nread;
 	while (size() < required_size) {
 		char buffer[SOCKET_READ_SIZE];
 		
+		ssize_t nread;
 		if ((ret = skt->read(buffer, SOCKET_READ_SIZE, &nread)) != ERROR_SUCCESS) {
 			return ret;
 		}