BUG/MINOR: config: check capture pool creations for failures

BUG/MINOR: config: check capture pool creations for failures

A few capture pools can fail in case of too large values for example.
These include the req_uri, capture, and caphdr pools, and may be triggered
with "tune.http.logurilen 2147483647" in the global section, or one of
these in a frontend:

  capture request header name len 2147483647
  http-request capture src len 2147483647
  tcp-request content capture src len 2147483647

These seem to be the only occurrences where create_pool()'s return value
is assigned without being checked, so let's add the proper check for
errors there. This can be backported as a hardening measure though the
risks and impacts are extremely low.