Ensure that Checking_For_Potentially_Static_Expression flag is
manipulated in a stack-like manner (with stack depth 1 at the most). The
previous assertion didn't prevent us from setting the flag to True or to
False twice in a row.

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

gcc/ada/

	* sem_eval.adb (Set_Checking_Potentially_Static_Expression):
	Stronger assertion.