hevc: make the crop sizes unsigned

(cherry picked from commit c929659bdd)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/hevc.h

@@ -284,10 +284,10 @@ typedef struct RefPicListTab {
 } RefPicListTab;
 
 typedef struct HEVCWindow {
-    int left_offset;
-    int right_offset;
-    int top_offset;
-    int bottom_offset;
+    unsigned int left_offset;
+    unsigned int right_offset;
+    unsigned int top_offset;
+    unsigned int bottom_offset;
 } HEVCWindow;
 
 typedef struct VUI {