PR c++/83555
	* typeck.c (build_static_cast_1): For static casts to reference types,
	call build_base_path with flag_delete_null_pointer_checks as nonnull
	instead of always false.  When -fsanitize=null, call
	ubsan_maybe_instrument_reference on the NULL reference INTEGER_CST.
	* cp-gimplify.c (cp_genericize_r): Don't walk subtrees of UBSAN_NULL
	call if the first argument is INTEGER_CST with REFERENCE_TYPE.

	* g++.dg/opt/pr83555.C: New test.
	* g++.dg/ubsan/pr83555.C: New test.

From-SVN: r256186