ff_emulated_edge_mc: fix handling of w/h being 0

Fixes assertion failure

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dsputil_template.c

@@ -130,6 +130,9 @@ void FUNC(ff_emulated_edge_mc)(uint8_t *buf, const uint8_t *src, int linesize, i
     int x, y;
     int start_y, start_x, end_y, end_x;
 
+    if(!w || !h)
+        return;
+
     if(src_y>= h){
         src-= src_y*linesize;
         src+= (h-1)*linesize;

libavcodec/x86/dsputil_mmx.c

@@ -1675,6 +1675,9 @@ static av_always_inline void emulated_edge_mc(uint8_t *buf, const uint8_t *src,
 {
     int start_y, start_x, end_y, end_x, src_y_add = 0;
 
+    if(!w || !h)
+        return;
+
     if (src_y >= h) {
         src -= src_y*linesize;
         src_y_add = h - 1;