auto merge of #12302 : alexcrichton/rust/issue-12295, r=brson

auto merge of #12302 : alexcrichton/rust/issue-12295, r=brson

The previous code erroneously assumed that 'steals > cnt' was always true, but
that was a false assumption. The code was altered to decrement steals to a
minimum of 0 instead of taking all of cnt into account.

I didn't include the exact test from #12295 because it could run for quite
awhile, and instead set the threshold for MAX_STEALS to much lower during
testing. I found that this triggered the old bug quite frequently when running
without this fix.

Closes #12295