For a parser, the input buffer is always != NULL: In case of flushing,
the indicated size of the input buffer will be zero and the input buffer
will point to a zeroed buffer of size 0 + AV_INPUT_BUFFER_PADDING.
Therefore one does not need to check for whether said buffer is NULL or
not.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>