avformat/hlsenc: fix default AES key file url with variant streams

avformat/hlsenc: fix default AES key file url with variant streams

Currently when hls_enc is active and there are multiple variant stream
outputs, default key file url construction does not work, because it is
based on the FormatContext' url field. But in case of multiple variant
streams, it contains the variant m3u8 output playlist url that contains
the %v placeholder. So the result key file url will hold the %v
placeholder causing run time error message about "could not write the
key file".
This patch correct this behaviour, and use the master playlist url for
constructing the output key file url when master playlist is vailable.

Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>