avfilter/vf_unsharp: Don't dereference NULL

avfilter/vf_unsharp: Don't dereference NULL

The unsharp filter uses an array of arrays of uint32_t, each of which is
separately allocated. These arrays also need to freed separately; but
before doing so, one needs to check whether the array of arrays has
actually been allocated, otherwise one would dereference a NULL pointer.
This fixes #8408.

Furthermore, the array of arrays needs to be zero-initialized so that
no uninitialized pointer will be freed in case an allocation of one of
the individual arrays fails.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>