sftp_open: clean up, better check of input data

The clang-analyzer report made it look into this function and
I've went through it to remove a potential use of an
uninitialized variable and I also added some validation of input
data received from the server.

In general, lots of more code in this file need to validate the
input before assuming it is correct: there are servers out there
that have bugs or just have another idea of how to do the SFTP
protocol.

src/libssh2_priv.h

@@ -554,7 +554,7 @@ struct _LIBSSH2_SFTP_HANDLE
     unsigned char request_packet[SFTP_HANDLE_MAXLEN + 25];
 
     char handle[SFTP_HANDLE_MAXLEN];
-    int handle_len;
+    size_t handle_len;
 
     char handle_type;