BUG/MINOR: server: don't use "proxy" when px is really meant.

In server_parse_sni_expr(), we use the "proxy" global variable, when we
should probably be using "px" given as an argument.
It happens to work by accident right now, but may not in the future.

[wt: better backport it]
diff --git a/src/server.c b/src/server.c
index 878293f..a151fd4 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1539,10 +1539,10 @@
 	};
 
 	idx = 0;
-	proxy->conf.args.ctx = ARGC_SRV;
+	px->conf.args.ctx = ARGC_SRV;
 
 	expr = sample_parse_expr((char **)args, &idx, px->conf.file, px->conf.line,
-	                         err, &proxy->conf.args);
+	                         err, &px->conf.args);
 	if (!expr) {
 		memprintf(err, "error detected while parsing sni expression : %s", *err);
 		return ERR_ALERT | ERR_FATAL;