Regex: fix decoding of named capture indexes

Regex: fix decoding of named capture indexes

Previously, PCRE name-table capture indexes were decoded through plain char.
On signed-char targets, bytes with the high bit set were sign-extended and
produced a wild or incorrect capture index used to read a match array.

The fix is to decode unsigned bytes through a checked output parameter and to
validate every named capture index against the compiled capture count.

The issue was introduced in 9496944a (0.3.2).

This closes #1088 issue on GitHub.