vf_yadif: Fixed potential access violation on EOF.

The correct invariant is (cur != NULL -> next != NULL). Not the other way around.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_yadif.c

@@ -17,6 +17,7 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#include "libavutil/avassert.h"
 #include "libavutil/cpu.h"
 #include "libavutil/common.h"
 #include "libavutil/pixdesc.h"
@@ -238,6 +239,8 @@ static void start_frame(AVFilterLink *link, AVFilterBufferRef *picref)
     AVFilterContext *ctx = link->dst;
     YADIFContext *yadif = ctx->priv;
 
+    av_assert0(picref);
+
     if (yadif->frame_pending)
         return_frame(ctx, 1);
 
@@ -308,7 +311,7 @@ static int request_frame(AVFilterLink *link)
 
         ret  = avfilter_request_frame(link->src->inputs[0]);
 
-        if (ret == AVERROR_EOF && yadif->next) {
+        if (ret == AVERROR_EOF && yadif->cur) {
             AVFilterBufferRef *next = avfilter_ref_buffer(yadif->next, AV_PERM_READ);
             next->pts = yadif->next->pts * 2 - yadif->cur->pts;