swscale/utils: Clear pix buffers

Fixes use of uninitialized memory
Fixes: a96874b9466b6edc660a519c7ad47977_signal_sigsegv_7ffff713351a_744_nc_sample.avi with memlimit 2147483648

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
