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_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fb55e95396caa5f03dc37f72522313eb5c095ee1

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