ffv1: Make sure at least one slice context is initialized

This avoids crashes when initializing the range coder for
the first slice context.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/ffv1.c

@@ -194,6 +194,10 @@ av_cold int ffv1_init_slice_contexts(FFV1Context *f)
     int i;
 
     f->slice_count = f->num_h_slices * f->num_v_slices;
+    if (f->slice_count <= 0) {
+        av_log(f->avctx, AV_LOG_ERROR, "Invalid number of slices\n");
+        return AVERROR(EINVAL);
+    }
 
     for (i = 0; i < f->slice_count; i++) {
         FFV1Context *fs = av_mallocz(sizeof(*fs));