Avoid null pointer dereference crash after OOM in Snowball stemmers.

Avoid null pointer dereference crash after OOM in Snowball stemmers.

Absorb upstream bug fix (their commit
e322673a841d9abd69994ae8cd20e191090b6ef4), which prevents a null
pointer dereference crash if SN_create_env() gets a malloc failure
at just the wrong point.

Thanks to Maksim Korotkov for discovering the null-pointer
bug and submitting the fix to upstream snowball.

Reported-by: Maksim Korotkov <m.korotkov@postgrespro.ru>
Author: Maksim Korotkov <m.korotkov@postgrespro.ru>
Discussion: http://postgr.es/m/1d1a46-67ab1000-21-80c451@83151435
Backpatch-through: 13