UObject: Fix SEGV when a threaded call is stopped by a tag.
UObject: Fix SEGV when a threaded call is stopped by a tag.

There was a small glitch in the tag-stop.chk test, that was supposed to test
exactly that, preventing us to detect the issue.

	* src/kernel/uobject.cc (uwrap_callbac, write_and_unfreeze): Handle
	the two exit cases, no race, no usage of shared ptr from an other
	thread.

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