Comment 2 for bug 960020

StacktraceTop:
 CopyFromUswc (dst=0x7f65f40125d0 "--", dst_pitch=864, src=0x7f660092e352 "--.//023566777777789:;<<====>>>>>>>>???@@@@AAAAABBCDFGGHHHGGFFEDCBBBBBCDDEFFGGGFEDDDEEEEDDEEFFGGGFDBBAAA@@ABBCDDCCBBAA?>=<;;;;;:7555555456888777666666789:::8879<===<<<<<<==>>?ADFFFFFFGGGDDDDEEEEEEEEEE"..., src_pitch=850, width=850, height=4, cpu=4072) at copy.c:86
 CopyPlane (dst=0x7f65e7f6a000 "", dst_pitch=852, src=0x7f660092e000 "(((((((())))))))****++,-.///--,,44320//////0000/.---666666554432/-----./012333432112222221223344431//./01221/.--,,,,,,,,+++++++***))))*+--+('&%&()))))((('''&&&(,...--,,++++,,-.122222334411112222222211"..., src_pitch=850, cache=0x7f65f4012270 "(((((((())))))))****++,-.///--,,44320//////0000/.---666666554432/-----./012333432112222221223344431//./01221/.--,,,,,,,,+++++++***))))*+--+('&%&()))))((('''&&&(,...--,,++++,,-.122222334411112222222211"..., cache_size=<optimized out>, width=850, height=480, cpu=4072) at copy.c:243
 CopyFromNv12 (dst=0x7f65e80026f0, src=0x7f663e47db90, src_pitch=0x7f663e47dbb0, width=<optimized out>, height=480, cache=0x7f65f40009e0) at copy.c:315
 Extract (p_external=0x7f65f40008c0, p_picture=0x7f65e80026f0, p_ff=<optimized out>) at vaapi.c:416
 vlc_va_Extract (va=<optimized out>, src=0x7f6608c9baa0, dst=0x7f65e80026f0) at va.h:54