[Ada] Null procedures not allowed in protected definitions

[Ada] Null procedures not allowed in protected definitions

The syntax rules do not allow null procedures in protected definitions. This
patch fixes a bug that accidentally allowed them.

2018-01-11  Bob Duff  <duff@adacore.com>

gcc/ada/

	* par-ch9.adb (P_Protected_Operation_Declaration_Opt): Give an error if
	a null procedure occurs in a protected definition.

gcc/testsuite/

	* gnat.dg/protected_null.adb: New testcase.

From-SVN: r256511