avfilter/vf_alphamerge: Fix double-free of AVFilterFormats on error

avfilter/vf_alphamerge: Fix double-free of AVFilterFormats on error

The query_formats function of the alphamerge filter tries to allocate
two lists of formats which on success are attached to more permanent
objects (AVFilterLinks) for storage afterwards. If attaching a list
to an AVFilterLink succeeds, the link becomes one of the owners of
the list. Yet if attaching a list to one of its links succeeds and
an error happens lateron, both lists were manually freed, which is wrong
if the list is already owned by one or more links; these links' pointers
to their lists will become dangling and there will be a double-free/use-
after-free when these links are cleaned up automatically.

This commit fixes this by removing the custom freeing code; this will
temporarily add a leaking codepath (if attaching a list not already
owned by a link to a link fails, the list will leak), but this will
be fixed soon by making sure that an AVFilterFormats without owner will
be automatically freed when attaching it to an AVFilterLink fails.
At most one list leaks because as of this commit a new list is only
allocated after the old list has been successfully attached to a link.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>