avfilter/unsharp_opencl: use av_mallocz_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_vignette: Use av_malloc_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vsrc_cellauto: Use av_mallocz_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>