dpx: Make start offset unsigned

Some corrupted files would end up with a negative offset,
and segfault.

Fixes bug #177.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/dpx.c

@@ -62,7 +62,8 @@ static int decode_frame(AVCodecContext *avctx,
     AVFrame *const p = &s->picture;
     uint8_t *ptr;
 
-    int magic_num, offset, endian;
+    unsigned int offset;
+    int magic_num, endian;
     int x, y;
     int w, h, stride, bits_per_color, descriptor, elements, target_packet_size, source_packet_size;