lib: Use nullptr as sentinel instead of NULL
lib: Use nullptr as sentinel instead of NULL

On some non-GCC/GLIBC systems, NULL may not be a suitable 0 pointer constant.

https://bugzilla.gnome.org/show_bug.cgi?id=762863
(cherry picked from commit e3d9eef71fe29f658d1376a215b02cfddc7a68bc)

saitoha/vte-sixel,thestinger/vte-ng,thestinger/vte-ng,saitoha/vte-sixel,saitoha/vte-sixel,thestinger/vte-ng