When using trap handlers, memory references do not get any checks inserted. This
means there is no check for a null memory as happens when the memory size is
0. Normally this would be correctly caught as an out of bounds access, since the
low memory addresses are not normally mapped. However, if they were mapped for
some reason, we would not catch the out of bounds access.

The fix is to ensure WebAssembly instances always have a guard region even if
the memory is size 0.

Bug: chromium:769637
Change-Id: I2d0f8c107563236c3780eb7746c2f820e319c65f
Reviewed-on: https://chromium-review.googlesource.com/693137Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48240}