shell: Check more thoroughly for undefined surface resource

shell: Check more thoroughly for undefined surface resource

The surface can have an undefined resource in certain situations (such
as with xwayland).  So, since NULL is a valid state for this parameter,
and since the wl_resource_*, etc. calls require their parameters to be
non-NULL, make a practice of always checking the surface resource before
making wayland calls.

update v2:
  * Fix some c/p errors for pointer names
  * Drop null ptr check in add_popup_grab; probably redundant now

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>