Validate the typed data before dereferencing the internal struct.
	 (#8315)

	---
	 process.c                  |  5 +++--
	 test/fiber/test_process.rb | 21 +++++++++++++++++++++
	 2 files changed, 24 insertions(+), 2 deletions(-)

In a forked process from a fiber, the fiber becomes the only
fiber, `fiber_switch` does nothing as there is no other fibers,
`rb_fiber_terminate` does not terminate the fiber.  In that case,
reaches the end of `fiber_entry` finaly, which is declared as
"COROUTINE" and should never return.