[St] Don't leak the font family name

A copy of the string is made by pango_font_description_set_family()
and we don't need the string anymore, so we should free it

https://bugzilla.gnome.org/show_bug.cgi?id=605035

src/st/st-theme-node.c

@@ -1953,7 +1953,10 @@ st_theme_node_get_font (StThemeNode *node)
     }
 
   if (family)
+    {
       pango_font_description_set_family (node->font_desc, family);
+      g_free (family);
+    }
 
   if (size_set)
     pango_font_description_set_absolute_size (node->font_desc, size);