BUG/MINOR: http-ana: Don't crush stream termination condition on internal error

BUG/MINOR: http-ana: Don't crush stream termination condition on internal error

When internal error is reported from an HTTP analyzer, we must take care to
not set the stream termination condition if it was already set. For
instance, it happens when a message rewrite fails. In this case
SF_ERR_PXCOND is set by the rule. The HTTP analyzer must not crush it with
SF_ERR_INTERNAL.

The regression was introduced with the commit 0fd25514d6 ("MEDIUM: http-ana:
Set termination state before returning haproxy response").

The bug was discovered working in the issue #2568. It must be backported to
2.9.