rv34: fix invalid memory access for small video dimensions

For small video dimensions calculations of the upper bound for pixel
access may result in negative value. Using an unsigned comparison
works only if the greater operand is non-negative. This is fixed by
doing edge emulation explicitly for such conditions.
Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>

libavcodec/rv34.c

@@ -680,8 +680,9 @@ static inline void rv34_mc(RV34DecContext *r, const int block_type,
     srcY += src_y * s->linesize + src_x;
     srcU += uvsrc_y * s->uvlinesize + uvsrc_x;
     srcV += uvsrc_y * s->uvlinesize + uvsrc_x;
-    if(   (unsigned)(src_x - !!lx*2) > s->h_edge_pos - !!lx*2 - (width <<3) - 4
-       || (unsigned)(src_y - !!ly*2) > s->v_edge_pos - !!ly*2 - (height<<3) - 4){
+    if(s->h_edge_pos - (width << 3) < 6 || s->v_edge_pos - (height << 3) < 6 ||
+       (unsigned)(src_x - !!lx*2) > s->h_edge_pos - !!lx*2 - (width <<3) - 4 ||
+       (unsigned)(src_y - !!ly*2) > s->v_edge_pos - !!ly*2 - (height<<3) - 4) {
         uint8_t *uvbuf = s->edge_emu_buffer + 22 * s->linesize;
 
         srcY -= 2 + 2*s->linesize;