videodsp: don't overread edges in vfix3 emu_edge.

Fixes trac ticket 3226. Also see Andreas' analysis in
https://bugs.debian.org/801745, which was very helpful.

libavcodec/x86/videodsp.asm

@@ -194,8 +194,12 @@ hvar_fn
 %elif (%2-%%off) == 2
     mov            valw, [srcq+%2-2]
 %elifidn %1, body
-    mov            vald, [srcq+%2-3]
-%else
+    mov            valb, [srcq+%2-1]
+    sal            vald, 16
+    mov            valw, [srcq+%2-3]
+%elifidn %1, bottom
+    movd mm %+ %%mmx_idx, [srcq+%2-4]
+%else ; top
     movd mm %+ %%mmx_idx, [srcq+%2-3]
 %endif
 %endif ; (%2-%%off) >= 1
@@ -251,12 +255,15 @@ hvar_fn
     mov     [dstq+%2-2], valw
 %elifidn %1, body
     mov     [dstq+%2-3], valw
-    shr            vald, 16
+    sar            vald, 16
     mov     [dstq+%2-1], valb
 %else
     movd           vald, mm %+ %%mmx_idx
+%ifidn %1, bottom
+    sar            vald, 8
+%endif
     mov     [dstq+%2-3], valw
-    shr            vald, 16
+    sar            vald, 16
     mov     [dstq+%2-1], valb
 %endif
 %endif ; (%2-%%off) >= 1