BUG/MINOR: mux-fcgi: Correctly set pathinfo

BUG/MINOR: mux-fcgi: Correctly set pathinfo

Existing logic for checking whether a regex subexpression for pathinfo
is matched results in valid matches being ignored and non-matches having
a new zero length string stored in params->pathinfo. This patch reverses
the logic so params->pathinfo is set when the subexpression is matched.

Without this patch the example configuration in the documentation:

	path-info ^(/.+\.php)(/.*)?$

does not result in PATH_INFO being sent to the FastCGI application, as
expected, when the second subexpression is matched (in which case both
pmatch[2].rm_so and pmatch[2].rm_eo will be non-negative integers).

This patch may be backported as far as 2.2, the first release that made
the capture of this second subexpression optional.