demux_lavf: fix API usage

avio_alloc_context() is documented to require an av_malloc'ed buffer. It
appears libavformat can even reallocate the buffer while it is probing,
so passing a static buffer can in theory lead to crashes.

I couldn't reproduce such a crash, but apparently it happened to
mplayer-svn. This commit follows the mplayer fix in svn commit r36397.
demux_lavf: make avio buffer configurable

Perhaps not very useful, but reserved for situations when a user reports
awful latency and experimentation/debugging might be required to find
out why or to fix it (happens often).