[Ada] Prevent crashes when pretty-printing freeze nodes from gdb

[Pierre-Marie de Rodat]

With recently enabled data validity checking in development builds
routine "ps", when called from gdb to examine the AST, might crash on
invalid data.  This is because its Sprint_Node_Actual callee reads
Dump_Freeze_Null global variable when processing nodes of
N_Freeze_Entity and this global variable is not initialized by default.

Found while working on inheritance of the Default_Initial_Condition
pragma.  Compilation is not affected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sprint.adb (po): Set Dump_Freeze_Null to False; align colons.
	(ps): Likewise.
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