BUG/MEDIUM: http-act: Don't replace URI if path is not found or invalid

BUG/MEDIUM: http-act: Don't replace URI if path is not found or invalid

For replace-path, replace-pathq and replace-uri actions, we must take care
to not match on the selected element if it is not defined.

regex_exec_match2() function expects to be called with a defined
subject. However, if the request path is invalid or not found, the function
is called with a NULL subject, leading to a crash when compiled without the
PRCE/PCRE2 support.

For instance the following rules crashes HAProxy on a CONNECT request:

  http-request replace-path /short/(.) /\1

This patch must be backported as far as 2.0.