ada: Constraint error not raised in ACATS test c413007

Reverse the meaning of switch -gnatd_P; that is, enable by default
the generating of a runtime check when the prefix of the call is
an access-to-subprogram type with a null value.

gcc/ada/ChangeLog:

	* sem_res.adb (Resolve_Actuals): Add by default a null-exclusion
	check on the prefix of the call when it is an access-type; it can
	be disabled using -gnatd_P.
	* debug.adb (gnatd_P): Update documentation.