ada: Fix another minor fallout of previous changes to aggregate expansion

This is another glitch associated with Initialization_Statements.

gcc/ada/ChangeLog:

	* exp_util.adb (Remove_Init_Call): Rewrite a compound statement in
	the Initialization_Statements of the variable as a null statement
	instead of removing it.
	* freeze.adb (Explode_Initialization_Compound_Statement): Small
	comment tweaks.