[Ada] Spurious constraint error on conversion of access types

[Pierre-Marie de Rodat]

This patch fixes an error in the compiler whereby a spurious constraint
error is raised at runtime on type conversions between
access-to-discriminanted types when the object being converted is null.

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

gcc/ada/

	* checks.adb (Make_Discriminant_Constraint_Check): Add check for
	null when the type being converted is an access type.
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