Fix av_open_input_stream with uninitialized context pointer.

Code would allocate a new context but forget to assign it
to the pointer actually passed to avformat_open_input,
potentially causing a crash.
Even if it was initialized it would cause a memleak.
This caused crashes with e.g. mpd, see also
http://bugs.gentoo.org/show_bug.cgi?id=373423

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
