avcodec/bmp: Fix runtime error: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself

There is code checking height and width later, leaving an invalid value invalid
is thus fine.

Fixes: 635/clusterfuzz-testcase-6225161437052928
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/bmp.c

@@ -133,7 +133,7 @@ static int bmp_decode_frame(AVCodecContext *avctx,
     }
 
     avctx->width  = width;
-    avctx->height = height > 0 ? height : -height;
+    avctx->height = height > 0 ? height : -(unsigned)height;
 
     avctx->pix_fmt = AV_PIX_FMT_NONE;