avcodec/parser: Don't return pointer to stack buffer

avcodec/parser: Don't return pointer to stack buffer

When flushing, the parser receives a dummy buffer with padding
that lives on the stack of av_parser_parse2(). Certain parsers
(e.g. Dolby E) only analyze the input, but don't repack it. When
flushing, such parsers return a pointer to the stack buffer and
a size of 0. And this is also what av_parser_parse2() returns.

Fix this by always resetting poutbuf in case poutbuf_size is zero.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>