[Ada] Fix for resolution of overloaded subprogram for Iterable aspect

[Pierre-Marie de Rodat <derodat@adacore.com>]

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When resolving the Iterable aspect we look for a functions that are
declared in the same scope as the annotated type and that have the
required number and types formal parameters. However, we didn't guard
against functions that have no formal parameter at all.

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

gcc/ada/

	* sem_ch13.adb (Resolve_Iterable_Operation): Add guard to
	prevent crash when the examined function has no formal
	parameters and Etype is called on Empty entity.

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diff --git a/gcc/ada/sem_ch13.adb b/gcc/ada/sem_ch13.adb
--- a/gcc/ada/sem_ch13.adb
+++ b/gcc/ada/sem_ch13.adb
@@ -15943,6 +15943,7 @@ package body Sem_Ch13 is
             while Present (It.Typ) loop
                if Ekind (It.Nam) = E_Function
                   and then Scope (It.Nam) = Scope (Typ)
+                  and then Present (First_Formal (It.Nam))
                   and then Etype (First_Formal (It.Nam)) = Typ
                then
                   F1 := First_Formal (It.Nam);