BUG/MEDIUM: fcgi-app: fix memory leak in fcgi_flt_http_headers

BUG/MEDIUM: fcgi-app: fix memory leak in fcgi_flt_http_headers

When the loop is continued early, the memory for param_rule is not freed. This
can leak memory per request, which will eventually consume all available memory
on the server.

This patch should fix the issue #750. It must be backported as far as 2.1.

(cherry picked from commit b605a736b0766030746b50c322879d929fcebacf)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 4f472bedc8c696f9c18da0eb16904a98a37f076c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>