A local build with g++ 8.3.0 failed with two warnings. This CL fixes
them.
The errors were:

1) error: 'char* strncat(char*, const char*, size_t)' output truncated
   before terminating nul copying as many bytes from a string as its
   length [-Werror=stringop-truncation]
2) error: 'char* strncpy(char*, const char*, size_t)' specified bound
   depends on the length of the source argument
   [-Werror=stringop-overflow=]

R=petermarshall@chromium.org

Bug: v8:9396
Change-Id: I175a72435becd694e4151a64e538084ec17d8500
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1803636Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63788}