Avoid possible segfault in psql's tab completion.

Avoid possible segfault in psql's tab completion.

Fix oversight in bd1276a3c: the "words_after_create" stanza in
psql_completion() requires previous_words_count > 0, since it uses
prev_wd.  This condition was formerly assured by the if-else chain
above it, but no more.  If there were no previous words then we'd
dereference an uninitialized pointer, possibly causing a segfault.

Report and patch by Anthonin Bonnefoy.

Discussion: https://postgr.es/m/CAO6_XqrSRE7c_i+D7Hm07K3+6S0jTAmMr60RY41XzaA29Ae5uA@mail.gmail.com