Use checks for signed pointers and fix a fp-print property set

Remove the -Wno-pointer-sign flag and use appropriate types or casting when needed.

Also, when changing the dependency on glib-2.58, we have some nicer incompatible-pointer-types check when using g_clear_pointer and thanks to that, I've spotted an error we had when setting PROP_FPI_DATA, as we were clearing the invalid pointer with very possible memory errors.