BUG: MINOR: http: don't check http-request capture id when len is provided

BUG: MINOR: http: don't check http-request capture id when len is provided

Randomly, haproxy could fail to start when a "http-request capture"
action is defined, without any change to the configuration. The issue
depends on the memory content, which may raise a fatal error like :
  unable to find capture id 'xxxx' referenced by http-request capture
rule

Commit fd608dd2 already prevents the condition to happen, but this one
should be included for completeness and to reclect the code on the
response side.

The issue was introduced recently by commit 29730ba5 and should only be
backported to haproxy 1.8.