lavfi/avf_concat: fix invalid exclusive test.

The invalid test did not cause any actual problem since
the first branch is only possible with bogus filters.

Fix coverity issue CID 733850.

libavfilter/avf_concat.c

@@ -195,7 +195,7 @@ static void process_frame(AVFilterLink *inlink, AVFilterBufferRef *buf)
         av_log(ctx, AV_LOG_ERROR, "Frame after EOF on input %s\n",
                ctx->input_pads[in_no].name);
         avfilter_unref_buffer(buf);
-    } if (in_no >= cat->cur_idx + ctx->nb_outputs) {
+    } else if (in_no >= cat->cur_idx + ctx->nb_outputs) {
         ff_bufqueue_add(ctx, &cat->in[in_no].queue, buf);
     } else {
         push_frame(ctx, in_no, buf);