NULL-terminate shapers list
NULL-terminate shapers list

hb_shape_full() takes a NULL-terminated array (the length of the array
is not passed separately) and would otherwise try to access random
memory and crash if all shapers failed.

Fixes https://github.com/harfbuzz/uharfbuzz/issues/83

harfbuzz/uharfbuzz,harfbuzz/uharfbuzz