hnm4: change width/height to int to fix hypothetical integer overflows

CC: libav-stable@libav.org
Bug-Id: CID 1135770 / CID 1135771

libavcodec/hnm4video.c

@@ -37,8 +37,8 @@
 
 typedef struct Hnm4VideoContext {
     uint8_t version;
-    uint16_t width;
-    uint16_t height;
+    int width;
+    int height;
     uint8_t *current;
     uint8_t *previous;
     uint8_t *buffer1;