BUG/MEDIUM: http_ana: fix potential NULL deref in http_process_req_common()

BUG/MEDIUM: http_ana: fix potential NULL deref in http_process_req_common()

As reported by @kenballus in GH #3118, a potential NULL-deref was
introduced in 3da1d63 ("BUG/MEDIUM: http_ana: handle yield for "stats
http-request" evaluation")

Indeed, px->uri_auth may be NULL when stats directive is not involved in
the current proxy section.

The bug went unnoticed because it didn't seem to cause any side-effect
so far and valgrind didn't catch it. However ASAN did, so let's fix it
before it causes harm.

It should be backported with 3da1d63.