c++: Fix null deref in maybe_diagnose_standard_trait [PR121859]

c++: Fix null deref in maybe_diagnose_standard_trait [PR121859]

A static member template doesn't always have a DECL_INITIAL, as in the
below testcase, and so checking its TREE_CODE performs a null-pointer
dereference.  I don't think we need to specially detect this case as
traits we care about are unlikely to be members, so this just adds the
missing null check.

	PR c++/121859

gcc/cp/ChangeLog:

	* constraint.cc (maybe_diagnose_standard_trait): Check if expr
	is non-null.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-traits5.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>