vf_drawtext: don't give up its own reference to the input buffer.

Directly forwarding the input buffer to the next filter means that
drawtext no longer owns any references to it and thus shouldn't refer to
it in any way.

libavfilter/vf_drawtext.c

@@ -812,6 +812,7 @@ static void start_frame(AVFilterLink *inlink, AVFilterBufferRef *inpicref)
 {
     AVFilterContext *ctx = inlink->dst;
     DrawTextContext *dtext = ctx->priv;
+    AVFilterBufferRef *buf_out;
     int fail = 0;
 
     if (dtext_prepare_text(ctx) < 0) {
@@ -850,7 +851,8 @@ static void start_frame(AVFilterLink *inlink, AVFilterBufferRef *inpicref)
             (int)dtext->var_values[VAR_N], dtext->var_values[VAR_T],
             dtext->x, dtext->y, dtext->x+dtext->w, dtext->y+dtext->h);
 
-    ff_start_frame(inlink->dst->outputs[0], inpicref);
+    buf_out = avfilter_ref_buffer(inpicref, ~0);
+    ff_start_frame(inlink->dst->outputs[0], buf_out);
 }
 
 static void end_frame(AVFilterLink *inlink)
@@ -866,6 +868,7 @@ static void end_frame(AVFilterLink *inlink)
 
     ff_draw_slice(outlink, 0, picref->video->h, 1);
     ff_end_frame(outlink);
+    avfilter_unref_buffer(inlink->cur_buf);
 }
 
 AVFilter avfilter_vf_drawtext = {