Avoid fetching past the end of the indoption array.

Avoid fetching past the end of the indoption array.

pg_get_indexdef_worker carelessly fetched indoption entries even for
non-key index columns that don't have one.  99.999% of the time this
would be harmless, since the code wouldn't examine the value ... but
some fine day this will be a fetch off the end of memory, resulting
in SIGSEGV.

Detected through valgrind testing.  Odd that the buildfarm's valgrind
critters haven't noticed.