j2kdec: Fix integer overflow leading to a segfault

Fixes Ticket776
Bug found by: Diana Elena Muscalu

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/j2k_dwt.c

@@ -321,7 +321,7 @@ int ff_j2k_dwt_init(DWTContext *s, uint16_t border[2][2], int decomp_levels, int
     int i, j, lev = decomp_levels, maxlen,
         b[2][2];
 
-    if (decomp_levels >= FF_DWT_MAX_DECLVLS)
+    if ((unsigned)decomp_levels >= FF_DWT_MAX_DECLVLS)
         return AVERROR_INVALIDDATA;
     s->ndeclevels = decomp_levels;
     s->type = type;