hls: fix a null pointer dereference

Not allocating the pls->ctx will crash in libavformat/hls.c:1410, where
it tries to dereference the field.

Sample: http://ec24.rtp.pt/liverepeater/rtpn.smil/playlist.m3u8

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
