The lookahead did not check whether there is actually a byte left to be
read. So if the i32 comparison was the last byte in the function body,
we would read out of memory.
This CL fixes that by introducing a separate {lookahead} method which
does the proper bounds check and the lookahead.

R=jkummerow@chromium.org

Bug: chromium:1014834, v8:9831
Change-Id: I6499ae3f2c57d38a8fcb587b99ae4a4a6c70e426
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1864939Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64335}