Remove Unnecessary Null check in xpointer.c

For https://bugzilla.gnome.org/show_bug.cgi?id=738052

superfluous check
xpointer : fixing Null Pointers

For https://bugzilla.gnome.org/show_bug.cgi?id=738053
At many places in xpointer.c
Null check is missing which is dereferenced at later places.