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

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.