When an xcb_connection_t goes into an error state, all operations on it will
fail. This means that after a call to xcb_key_symbols_get_reply(), syms->u.reply
would still be a NULL pointer and that xcb_get_setup() returns a NULL pointer.

The only way for xcb_get_setup() to return NULL is for an error connection, but
xcb_get_keyboard_mapping_reply() could also fail for other reasons. So to fix
this, all functions need to check for error connections and if syms->u.reply is
not NULL.

This was tested with the following C code:

  #include <xcb_keysyms.h>
  #include <stdio.h>
  #include <stdlib.h>

  int main()
  {
  	xcb_connection_t *c = xcb_connect(NULL, NULL);
  	xcb_key_symbols_t *syms = xcb_key_symbols_alloc(c);
  	/* The above sent a GetKeyboardMapping request. Let's now break the
  	 * connection so that it cannot get the reply.
  	 */
  	uint32_t max = xcb_get_maximum_request_length(c);
  	xcb_screen_t *s = xcb_setup_roots_iterator(xcb_get_setup(c)).data;
  	size_t l...