[PATCH] Don't modify the DenseMap being iterated over from within the loop

[Duncan P. N. Exon Smith]

> On 2015 Feb 26, at 17:45, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> 
>> If you have a test case that happens to tickle this, always nice to add.
> 
> No, unfortunately.  I am playing around with making the DenseMap and
> DenseSet iterators "fail-fast" (coming soon, watch this space!) and
> this one popped out.

Nice!