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/legacy.c

@@ -145,7 +145,7 @@ void buffer_free(ssh_buffer buffer){
   ssh_buffer_free(buffer);
 }
 void *buffer_get(ssh_buffer buffer){
-  return ssh_buffer_get_begin(buffer);
+  return ssh_buffer_get(buffer);
 }
 uint32_t buffer_get_len(ssh_buffer buffer){
   return ssh_buffer_get_len(buffer);