R=rossberg@chromium.org

Bug: 
Change-Id: Icac33dc87dd660173e5a45d02b31be46f7d1cb2d
Reviewed-on: https://chromium-review.googlesource.com/721550
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48632}

Currently when the module has memory specified in the compiled bytes, but with no initial memory
exported memory assigns a bogus buffer to the instance. When grow_memory is called on this buffer, it tries to patch an incorrect address.
 - Fix exported memory to handle no initial memory
 - Fix grow_memory to handle uninitialized buffers

BUG=chromium:710844
R=bradnelson@chromium.org

Review-Url: https://codereview.chromium.org/2820223002
Cr-Commit-Position: refs/heads/master@{#44671}