imgutils: initialize palette padding bytes in av_image_alloc

av_image_fill_pointers always aligns the palette, but the padding
bytes don't (and can't) get initialized in av_image_copy.

Thus initialize them in av_image_alloc.

This fixes 'Syscall param write(buf) points to uninitialised byte(s)'
valgrind warnings.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
