Prevent NULL dereferences when missing the reference frame in the Electronic Arts CMV decoder.

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

libavcodec/eacmv.c

@@ -110,6 +110,7 @@ static void cmv_decode_inter(CmvContext * s, const uint8_t *buf, const uint8_t *
         }else{  /* inter using last frame as reference */
             int xoffset = (buf[i] & 0xF) - 7;
             int yoffset = ((buf[i] >> 4)) - 7;
+            if (s->last_frame.data[0])
                 cmv_motcomp(s->frame.data[0], s->frame.linesize[0],
                           s->last_frame.data[0], s->last_frame.linesize[0],
                           x*4, y*4, xoffset, yoffset, s->avctx->width, s->avctx->height);