BUG/MINOR: http rule: http capture 'id' rule points to a non existing id

BUG/MINOR: http rule: http capture 'id' rule points to a non existing id

It is possible to create a http capture rule which points to a capture slot
id which does not exist.

Current patch prevent this when parsing configuration and prevent running
configuration which contains such rules.

This configuration is now invalid:

  frontend f
   bind :8080
   http-request capture req.hdr(User-Agent) id 0
   default_backend b

this one as well:

  frontend f
   bind :8080
   declare capture request len 32 # implicit id is 0 here
   http-request capture req.hdr(User-Agent) id 1
   default_backend b

It applies of course to both http-request and http-response rules.