BUG/MINOR: server: add missing free for server->rdr_pfx

rdr_pfx was not being free during server cleanup, leading to small memory
leak when "redir" argument was used on a server line (HTTP only).

This should be backported to every stable versions.

[For 2.6 and 2.7: the free should be performed in srv_drop() directly.
 For older versions: free in deinit() function near the free for the
 cookie string]

(cherry picked from commit 2c9bd3ae808872e52c07d7ec1d62f734dcbb6776)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/server.c b/src/server.c
index d2190de..af56266 100644
--- a/src/server.c
+++ b/src/server.c
@@ -2471,6 +2471,7 @@
 void srv_free_params(struct server *srv)
 {
 	free(srv->cookie);
+	free(srv->rdr_pfx);
 	free(srv->hostname);
 	free(srv->hostname_dn);
 	free((char*)srv->conf.file);