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
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/363382521eb26106587c7dab7f439636c3a6876a

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