avfilter/af_join: Fix crash in join filter

Previously if ff_outlink_frame_wanted() returned 0 it could dereference a null pointer when trying to read nb_samples.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/af_join.c

@@ -485,6 +485,9 @@ static int activate(AVFilterContext *ctx)
                 return 0;
             }
         }
+        if (!s->input_frames[0]) {
+            return 0;
+        }
     }
 
     nb_samples = s->input_frames[0]->nb_samples;