BUG/MEDIUM: http-ana: fix crash or wrong header deletion by http-restrict-req-hdr...

BUG/MEDIUM: http-ana: fix crash or wrong header deletion by http-restrict-req-hdr-names

When using `option http-restrict-req-hdr-names delete`, HAproxy may
crash or delete wrong header after receiving request containing multiple
forbidden characters in single header name; exact behavior depends on
number of request headers, number of forbidden characters and position
of header containing them.

This patch fixes GitHub issue #1822.

Must be backported as far as 2.2 (buggy feature got included in 2.2.25,
2.4.18 and 2.5.8).