[Ada] Crash in interface derivation with null primitive

[Ada] Crash in interface derivation with null primitive

The frontend crashes processing the derivation of a tagged type whose
ultimate ancestor is an interface type I1 that has a null primitive,
implements another interface I2 derived from I2, and does not override
the null primitive.

2019-07-08  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* exp_disp.adb (Register_Primitive): When registering a
	primitive in the secondary dispatch table, handle primitive
	inherited through several levels of type derivation (required to
	properly handle inherited 'null' primitive).

gcc/testsuite/

	* gnat.dg/interface9.adb, gnat.dg/interface9_root-child.ads,
	gnat.dg/interface9_root.ads: New testcase.

From-SVN: r273204