graphparser: add missing NULL check in avfilter_graph_parse()

Fix a crash occurring when open_inputs is NULL and *open_inputs is
checked, the crash was introduced by the recent avfilter_graph_parse()
syntax change.

In particular, fix graph2dot crash.

libavfilter/graphparser.c

@@ -387,7 +387,7 @@ int avfilter_graph_parse(AVFilterGraph *graph, const char *filters,
         goto fail;
     }
 
-    if (*open_inputs && !strcmp((*open_inputs)->name, "out") && curr_inputs) {
+    if (open_inputs && *open_inputs && !strcmp((*open_inputs)->name, "out") && curr_inputs) {
         /* Last output can be omitted if it is "[out]" */
         const char *tmp = "[out]";
         if ((ret = parse_outputs(&tmp, &curr_inputs, open_inputs, open_outputs,