ada: Fix crash during function return analysis

The compiler would crash when checking type relation between the
function's return type and the type of the expression used in the return
statement. It would not work if the function's return type is an access
type and the expression is not.

gcc/ada/

	* sem_ch6.adb (Analyze_Function_Return): Add missing
	Is_Access_Type check before accessing the Designated_Type field.

Tested on x86_64-pc-linux-gnu, committed on master.

gcc/ada/sem_ch6.adb

@@ -838,6 +838,7 @@ package body Sem_Ch6 is
               and then Serious_Errors_Detected = 0
               and then Is_Access_Type (R_Type)
               and then Nkind (Expr) not in N_Null | N_Raise_Expression
+              and then Is_Access_Type (Etype (Expr))
               and then Is_Interface (Designated_Type (R_Type))
               and then Is_Progenitor (Designated_Type (R_Type),
                                       Designated_Type (Etype (Expr)))