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

src/backend/regex/regexec.c

@@ -200,6 +200,8 @@ pg_regexec(regex_t *re,
 		return REG_INVARG;
 	if (re->re_csize != sizeof(chr))
 		return REG_MIXED;
+	if (search_start > len)
+		return REG_NOMATCH;
 
 	/* Initialize locale-dependent support */
 	pg_set_regex_collation(re->re_collation);