glib: Various minor scan-build fixes

These squash various warnings from `scan-build`. None of them are
legitimate bugs, but some of them do improve code readability a bit.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1767

glib/gvariant-parser.c

@@ -2527,6 +2527,8 @@ g_variant_new_parsed_va (const gchar *format,
   if (*stream.stream)
     g_error ("g_variant_new_parsed: trailing text after value");
 
+  g_clear_error (&error);
+
   return result;
 }