BUG/MAJOR: http: fix bug in parse_qvalue() when selecting compression algo

Commit ad90351 ("MINOR: http: Add the "language" converter to for use with accept-language")
introduced a typo in parse_qvalue :

	if (*end)
		*end = qvalue;

while it should be :

	if (end)
		*end = qvalue;

Since end is tested for being NULL. This crashes when selecting the
compression algorithm since end is NULL here. No backport is needed,
this is just in latest 1.5-dev.
diff --git a/src/proto_http.c b/src/proto_http.c
index a22e250..1e038f4 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -2151,7 +2151,7 @@
  out:
 	if (q > 1000)
 		q = 1000;
-	if (*end)
+	if (end)
 		*end = qvalue;
 	return q;
 }