shell: assert get_shell_surface() != NULL as appropriate

Various functions that operate on a weston_surface assume the
surface has a shell_surface.  That is, they unconditionally
deref the get_shell_surface() result.  Hence, if for some reason
the call to get_shell_surface() returned NULL to those functions then
a segmentation fault would occur and the program would crash.  So,
adding an assert(...) on the get_shell_surface() return value adds an
extra sanity check and does not change this behavior.  The assert also
adds an extra benefit to the programmer by documenting that the function
expects and requires the weston_surface to have a shell_surface and
would be a program logic error, otherwise.

The assert() also silences some static analyzers about the possible
NULL deref.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>