[Ada] Fix crash when printing error message

[Pierre-Marie de Rodat]

Missing Chars on N were causing a crash - the solution is to set the
Sloc from N and to use F_type's chars.

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

gcc/ada/

        * freeze.adb (Freeze_Profile): Use N's Sloc, F_type's chars.
diff --git a/gcc/ada/freeze.adb b/gcc/ada/freeze.adb
--- a/gcc/ada/freeze.adb
+++ b/gcc/ada/freeze.adb
@@ -4141,9 +4141,10 @@ package body Freeze is
                elsif not After_Last_Declaration
                  and then not Freezing_Library_Level_Tagged_Type
                then
-                  Error_Msg_Node_1 := F_Type;
-                  Error_Msg_N
-                    ("type & must be fully defined before this point", N);
+                  Error_Msg_NE
+                    ("type & must be fully defined before this point",
+                     N,
+                     F_Type);
                end if;
             end if;