desktop-shell: don't crash if a surface disappears while grabbed

A surface can get destroyed while a shell grab is active, which can for example happen if the command running in weston-terminal exits. (Test this by opening weston-terminal, starting to interactively rotate it and pressing Ctrl-D to exit the shell.)

When a surface gets destroyed, grab->shsurf is reset to NULL by destroy_shell_grab_shsurf(), but otherwise the grab remains active and its callbacks continue to be called. Thus, dereferencing grab->shsurf in a callback without checking it for NULL first can lead to undefined behavior, including crashes.

Several functions were already properly checking grab->shsurf for NULL, move_grab_motion() being one example. Others, however, were not, which is what this commit fixes.

Related to #192 (closed), but does not actually include the patch from 79a09166 which fixes the same problem in resize_grab_button() and as the result fixes #192 (closed). It is expected that this gets applied on top of 79a09166 (though they don't conflict with each other and can be applied in any order).