VP6: fix vp6_filter_diag4_mmx/sse on 64-bit

The stride can be negative and must be sign extended before being
used in pointer arithmetic.

Originally committed as revision 24926 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/x86/vp56dsp.asm

@@ -127,6 +127,9 @@ cglobal vp6_filter_diag4_%1, 5, 7, %2
     sub         rsp, 8*15
     movq         m6, [pw_64]
 %endif
+%ifdef ARCH_X86_64
+    movsxd       r2, r2d
+%endif
 
     sub          r1, r2