avcodec/cinepak: fix integer underflow

Fixes out of array access
Fixes: asan_heap-oob_4da0ba_6_asan_heap-oob_4da0ba_241_cvid_crash.avi

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/cinepak.c

@@ -135,7 +135,7 @@ static int cinepak_decode_vectors (CinepakContext *s, cvid_strip *strip,
     const uint8_t   *eod = (data + size);
     uint32_t         flag, mask;
     uint8_t         *cb0, *cb1, *cb2, *cb3;
-    unsigned int     x, y;
+    int             x, y;
     char            *ip0, *ip1, *ip2, *ip3;
 
     flag = 0;