[Ada] Initialize Current_Error_Node

[Pierre-Marie de Rodat]

... because in case of compiler bugs, it can be accessed early, so it
needs to be initialized.

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

gcc/ada/

        * atree.ads (Current_Error_Node): Initialize to Empty.
diff --git a/gcc/ada/atree.ads b/gcc/ada/atree.ads
--- a/gcc/ada/atree.ads
+++ b/gcc/ada/atree.ads
@@ -130,7 +130,7 @@ package Atree is
    --  Current_Error_Node is also used for other purposes. See, for example,
    --  Rtsfind.
 
-   Current_Error_Node : Node_Id;
+   Current_Error_Node : Node_Id := Empty;
    --  Node to place compiler abort messages
 
    ------------------