avcodec/snow: split block clipping checks

Fixes out of array read
Fixes: d4476f68ca1c1c57afbc45806f581963-asan_heap-oob_2266b27_8607_cov_4044577381_snow_chroma_bug.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/snow.h

@@ -318,7 +318,8 @@ static av_always_inline void add_yblock(SnowContext *s, int sliced, slice_buffer
         if(!sliced && !offset_dst)
             dst -= src_x;
         src_x=0;
-    }else if(src_x + b_w > w){
+    }
+    if(src_x + b_w > w){
         b_w = w - src_x;
     }
     if(src_y<0){
@@ -327,7 +328,8 @@ static av_always_inline void add_yblock(SnowContext *s, int sliced, slice_buffer
         if(!sliced && !offset_dst)
             dst -= src_y*dst_stride;
         src_y=0;
-    }else if(src_y + b_h> h){
+    }
+    if(src_y + b_h> h){
         b_h = h - src_y;
     }