vf_deshake: set cur_buf to NULL.

Fix a segfault.

libavfilter/vf_deshake.c

@@ -435,6 +435,7 @@ static int end_frame(AVFilterLink *link)
     float alpha = 2.0 / deshake->refcount;
     char tmp[256];
 
+    link->cur_buf = NULL; /* it is in 'in' now */
     if (deshake->cx < 0 || deshake->cy < 0 || deshake->cw < 0 || deshake->ch < 0) {
         // Find the most likely global motion for the current frame
         find_motion(deshake, (deshake->ref == NULL) ? in->data[0] : deshake->ref->data[0], in->data[0], link->w, link->h, in->linesize[0], &t);