libsysprof-ui: ignore negative and 0 averages

This ensures that we only calculate averages for marks that have a valid
duration. We also need a new field for the number of averages we added
so that we don't skew the results.

src/libsysprof-ui/sysprof-ui-private.h

@@ -32,6 +32,7 @@ typedef struct
   gint64   max;
   gint64   min;
   gint64   avg;
+  guint64  avg_count;
 } SysprofMarkStat;
 
 SysprofMarkStat *_sysprof_mark_stat_new                   (const gchar           *name);