SFTP: limit write() to not produce overly large packets

sftp_write() now limits how much data it gets at a time even more
than before. Since this function creates a complete outgoing
packet based on what gets passed to it, it is crucial that it
doesn't create too large packets.

With this method, there's also no longer any problem to use very
large buffers in your application and feed that to libssh2. I've
done numerous tests now with uploading data over SFTP using 100K
buffers and I've had no problems with that.

src/sftp.c

@@ -1402,12 +1402,9 @@ static ssize_t sftp_write(LIBSSH2_SFTP_HANDLE *handle, const char *buffer,
     unsigned char *s, *data;
     int rc;
 
-    /* There's no point in us accepting a VERY large packet here since we
-       cannot send it anyway. We just accept 4 times the big size to fill up
-       the queue somewhat. */
-
-    if(count > (MAX_SSH_PACKET_LEN*4))
-        count = MAX_SSH_PACKET_LEN*4;
+    /* we limit this to just send a single SSH packet at a time */
+    if(count > 32500)
+        count = 32500;
 
     packet_len = handle->handle_len + count + 25;