[Ada] Prevent early exits without restoring a global variable

[Pierre-Marie de Rodat]

In Analyze_Case_Statement we save the current value of
Unblocked_Exit_Count with the intention to restore it on exit. However,
when returning early due to errors we failed to restore the previous
value. It seems cleaner to only modify this variable after the possible
early exits.

Mainly just a cleanup; behaviour on a valid code is unaffected.

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

gcc/ada/

	* sem_ch5.adb (Analyze_Case_Statement): Move modification of
	Unblocked_Exit_Count after early return statements; fix typo in
	comment.
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