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Signed-off-by: Martin Storsjö <martin@martin.st>

Found with a clang-scan report on http://fate.libav.org/csa/

It currently does the following:
1) get a zeroed audio buffer
2) copy some properties (but not the data) of the input buffer to it
3) pass this buffer to the output filter
This looks useless and is indeed not used by any filters, therefore
delete it.

Make ff_null_filter_samples() (just pass the buffer to the next filter)
the new default.

It's redundant, since the input buffer is passed as a parameter to the
filter_samples() callback, and can lead to stale pointers remaining on
the link.

Fixes a possible invalid write, found by Nicolas George.

This is more consistent with naming in the rest of Libav.

This is easier to follow than having them randomly scattered in
avfilter.c and defaults.c.