pygspawn: improve error checking

gspawn 'argv' and 'envp' parameters expect sequences of strings.  This
is enforced by checking that the passed argument is a sequence and that
each item returned from it is a string.

We do now, however, verify that each item can be successfully taken from
the sequence.  'os.environ' is an example of an object that passes
PySequence_Check() but fails to return objects from PySequence_ITEM().

Add a simple NULL check to avoid the crash.

https://bugzilla.gnome.org/show_bug.cgi?id=669594