pdf: build experimental import also on posix
[scan-build] Assigned value is garbage or undefined

Avoid using an undefined cy offset for broken rotate(a,x) transform.
[scan-build] Dereference of null pointer

Access to field 'app_data' results in a dereference of a null pointer
(loaded from variable 'dia_menu').
This could happen if an object would implement get_object_menu
but than return NULL from it.
[scan-build] Dereference of null pointer

Array access (via field 'handles') results in a null pointer dereference
But only if the logic would be screwed. Now the g_return_if_fail() is
more strict to handle that case, too.