[Ada] Race condition in Make_Independent

[Arnaud Charlet <charlet at adacore dot com>]

Make_Independent decrements Wait_Count, which caused a race condition -- if the
timing is just wrong, it causes Make_Passive to find Wait_Count = 0, which
eventually causes the program to hang. This patch prevents hanging by
decrementing Wait_Count only if nonzero.
No test is available, because it's large, and only fails intermittently.

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

2011-08-29  Bob Duff  <duff@adacore.com>

	* s-tasuti.adb (Make_Passive): Work around race condition in
	Make_Independent, which can cause Wait_Count to be zero. So instead of
	asserting that Wait_Count > 0, and then decrementing it, decrement it
	only if Wait_Count > 0.
	* s-taskin.ads (Wait_Count, Alive_Count, Awake_Count): All of these
	should be nonnegative, so declare them Natural instead of Integer.

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