Parsing labeled outputs involves a check for an already known match
(a labeled input with the same name) to pair them together. If yes,
it is attempted to create a link between the two filters; in this case
the AVFilterInOuts have fulfilled their purpose and are freed. Yet if
creating the link fails, these AVFilterInOuts have up until now not been
freed, although they had already been removed from their respective lists
(which means that they are not freed automatically). In other words:
They leak. This commit fixes this.

This fixes ticket #7084. Said ticket contains an example program to
reproduce a leak. It can also be reproduced with ffmpeg alone, e.g. with
the complex filters "[0]null[1],[2]anull[0]" or with "[0]abitscope[0]".
All of these three examples involve media type mismatches which make it
impossible to create the links. The bug could also be triggered by other
means, e.g. failure to allocate the necessary AVFilterLink.
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>