The logic for removing while iterating is non-standard and
a left over from a previous index based loop. This patch
replaces it with a standard erase based version.

This fixes a runtime crash with MSVC that invalidates the
iterator and then asserts. This also makes the code safe
in case the last move can be redundant.

Change-Id: Ie6990e0d65a3b83a4b7da3e2e89ed4e60a6cd215
Reviewed-on: https://chromium-review.googlesource.com/c/1488762Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59868}