lavf: sanity check size in av_get/append_packet().

To avoid allocating ridiculous amounts of memory for corrupted files,
read the input in chunks limited to filesize or an arbitrary large
amount when that is not known (chosen to be 50M).
lavfi: use designated initializers in avfilter_class
avfiltergraph: replace AVFilterGraph.filter_count with nb_filters

This is more consistent with the naming in the rest of Libav.
avfiltergraph: use sizeof(var) instead of sizeof(type)
filters.texi: restore mistakenly removed section name for noformat
AVOption: remove an unused function parameter.
AVOptions: fix using named constants with child contexts.

The named constant needs to be searched for in the same object on which
the option is set, i.e. target_obj.
vf_showinfo: remove its useless init function

Filter private data is memset to 0 so there is no point in explicitly
initing anything to 0.
asrc_anullsrc: do not set samplerate and channel layout explicitly

They are auto-negotiated.
asrc_anullsrc: return EOF, not -1
af_join: do not leak input frames.