copy_nonoverlapping panics on zero-sized NULL ptr copy

[RalfJung]

We somewhat recently declared copy_nonoverlapping to be legal on a NULL pointer. However, the debug assertions still reject this:

rust/library/core/src/intrinsics.rs

Lines 3315 to 3317 in 7caad69

	ub_checks::is_aligned_and_not_null(src, align)
	&& ub_checks::is_aligned_and_not_null(dst, align)
	&& ub_checks::is_nonoverlapping(src, dst, size, count)

Other, related functions have similar problems.

Cc @saethlin

[saethlin]

We have a very incomplete test suite for these checks here: https://github.com/rust-lang/rust/tree/master/tests/ui/precondition-checks

We should probably have been adding tests for every errant call that we detect.

[saethlin]

Started working on an implementation here: #131384