libavdevice/lavfi: check avfilter_graph_dump return value

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavdevice/lavfi.c

@@ -302,9 +302,13 @@ av_cold static int lavfi_read_header(AVFormatContext *avctx)
 
     if (lavfi->dump_graph) {
         char *dump = avfilter_graph_dump(lavfi->graph, lavfi->dump_graph);
-        fputs(dump, stderr);
-        fflush(stderr);
-        av_free(dump);
+        if (dump != NULL) {
+            fputs(dump, stderr);
+            fflush(stderr);
+            av_free(dump);
+        } else {
+            FAIL(AVERROR(ENOMEM));
+        }
     }
 
     /* fill each stream with the information in the corresponding sink */