If buffer_ is empty and start_ is zero, even though the MemCopy would be
a no-op, &buffer_[start_] is undefined. buffer_.data() + start_ would
work, but due to a C/C++ language bug, that is technically undefined
too[*] if an empty buffer_.data() returns nullptr, so add a length() ==
0 check, matching methods above.

This was caught by building with _LIBCPP_DEBUG=0.

[*] https://www.imperialviolet.org/2016/06/26/nonnull.html

Bug: chromium:893810
Change-Id: I9f0834ffae6769e0e191e786842e6ecc6c95a58a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1483616Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60016}