lavf/mux: improve feedback in case of no streams in muxer
doc/muxers/tee: use @ref to reference other sections
doc/muxers/tee: add example showing second level escaping
examples/decoding_encoding: check av_samples_get_buffer_size() for a negative value

Fix broken != 0 check.
vformat/id3v2: check avio_read for short reads in addition to errors

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