avcodec/iff: Use signed count

This is more a style fix than a bugfix (CID1604392 Overflowed constant)

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/iff.c

@@ -523,7 +523,7 @@ static int decode_byterun2(uint8_t *dst, int height, int line_size,
                            GetByteContext *gb)
 {
     GetByteContext cmds;
-    unsigned count;
+    int count;
     int i, y_pos = 0, x_pos = 0;
 
     if (bytestream2_get_be32(gb) != MKBETAG('V', 'D', 'A', 'T'))
@@ -531,7 +531,7 @@ static int decode_byterun2(uint8_t *dst, int height, int line_size,
 
     bytestream2_skip(gb, 4);
     count = bytestream2_get_be16(gb) - 2;
-    if (bytestream2_get_bytes_left(gb) < count)
+    if (count < 0 || bytestream2_get_bytes_left(gb) < count)
         return 0;
 
     bytestream2_init(&cmds, gb->buffer, count);