c: Don't warn about converting NULL to different sso endian [PR104822]

In a similar way we don't warn about NULL pointer constant conversion to
a different named address we should not warn to a different sso endian
either.
This adds the simple check.

Bootstrapped and tested on x86_64-linux-gnu with no regressions.

	PR c/104822

gcc/c/ChangeLog:

	* c-typeck.cc (convert_for_assignment): Check for null pointer
	before warning about an incompatible scalar storage order.

gcc/testsuite/ChangeLog:

	* gcc.dg/sso-18.c: New test.
	* gcc.dg/sso-19.c: New test.