Infinite loop in PrettyWriter

[Per Bothner]

Wen-Chun Ni wrote:
> When I was testing my program, certain string (FString really) will
> cause the program to hang.

I recently ran into a PrettyWriter hang, but haven't been able to
chase it down.  Your input helped me find one possible culprit.
Could you try the attached patch?  If it doesn't solve the problem,
I also added a bunch of comments that may help you understand
enough of how the queue works to debug it.
-- 
	--Per Bothner
per@bothner.com   http://www.bothner.com/per/
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