buffer: use ssh_buffer_get() instead of ssh_buffer_get_begin()

This commit is a preparatory stage for removing ssh_buffer_get_begin().
Note that removing ssh_buffer_get_begin() doesn't break API
compatibility, as this functions has never been exposed (it only has the
LIBSSH_API prefix).

Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/kex1.c

@@ -66,7 +66,7 @@ static ssh_string make_rsa1_string(ssh_string e, ssh_string n){
     goto error;
   }
 
-  ssh_string_fill(ret, ssh_buffer_get_begin(buffer), ssh_buffer_get_len(buffer));
+  ssh_string_fill(ret, ssh_buffer_get(buffer), ssh_buffer_get_len(buffer));
 error:
   ssh_buffer_free(buffer);
   ssh_string_free(rsa);