BUG/MINOR: http_ana: clarify connection pointer check on L7 retry

BUG/MINOR: http_ana: clarify connection pointer check on L7 retry

Coverity reports a possible null deref in issue #703. It seems this
cannot happen as in order to have a CF_READ_ERROR we'd need to have
attempted a recv() which implies a conn_stream, thus conn cannot be
NULL anymore. But at least one line tests for conn and the other one
not, which is confusing. So let's add a check for conn before
dereferencing it.

This needs to be backported to 2.1 and 2.0. Note that in 2.0 it's
in proto_htx.c.