libavfilter/graphparser: Avoid spaces in filter instance names.

This makes it easier to refer to filters where things are seperated by spaces.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/graphparser.c

@@ -99,7 +99,7 @@ static int create_filter(AVFilterContext **filt_ctx, AVFilterGraph *ctx, int ind
     char tmp_args[256];
     int ret;
 
-    snprintf(inst_name, sizeof(inst_name), "Parsed filter %d %s", index, filt_name);
+    snprintf(inst_name, sizeof(inst_name), "Parsed_%s_%d", filt_name, index);
 
     filt = avfilter_get_by_name(filt_name);