pgsql: Avoid platform-specific null pointer dereference in psql.

Avoid platform-specific null pointer dereference in psql.

POSIX permits getopt() to advance optind beyond argc when the last
argv entry is an option that requires an argument and hasn't got one.
It seems that no major platforms actually do that, but musl does,
so that something like "psql -f" would crash with that libc.
Add a check that optind is in range before trying to look at the
possibly-bogus option.

Report and fix by Quentin Rameau. Back-patch to all supported
branches.

Discussion: https://postgr.es/m/20190825100617.GA6087@fifth.space

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/65b1cad5a0929e685e79f7e84f56e0f28a54107a

Modified Files
--------------
src/bin/psql/startup.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)