pgsql: Make pg_regexec() robust against out-of-range search_start.

Make pg_regexec() robust against out-of-range search_start.

If search_start is greater than the length of the string, we should just
return REG_NOMATCH immediately. (Note that the equality case should
*not* be rejected, since the pattern might be able to match zero
characters.) This guards various internal assumptions that the min of a
range of string positions is not more than the max. Violation of those
assumptions could allow an attempt to fetch string[search_start-1],
possibly causing a crash.

Jaime Casanova pointed out that this situation is reachable with the
new regexp_xxx functions that accept a user-specified start position.
I don't believe it's reachable via any in-core call site in v14 and
below. However, extensions could possibly call pg_regexec with an
out-of-range search_start, so let's back-patch the fix anyway.

Discussion: https://postgr.es/m/20210911180357.GA6870@ahch-to

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3adde7eb6633bea734c4513ebd96adca78b7737c

Modified Files
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src/backend/regex/regexec.c | 2 ++
1 file changed, 2 insertions(+)