plug-ins, tiff: fix crash with clang when querying TIFFTAG_SUBIFD

The TIFFTAG_SUBIFD has a 16-bit length parameter, but we were using a
32-bit int that was not initialized to 0.
Apparently on clang when returning this value the high bits are not
set to 0.
This caused an incorrect high value which caused a stack overflow.
To fix this we change the type to gint16.

See issue #14432.

plug-ins/file-tiff/file-tiff-load.c

@@ -2324,7 +2324,7 @@ load_sketchbook_layers (TIFF      *tif,
   gchar         **image_settings;
   gchar          *hex_color;
   gint            layer_count = 0;
-  gint            sub_len;
+  gint16          sub_len;
   void           *ptr;
   gchar          *endptr = NULL;