src, buffer: do not segfault on out-of-range index

Also add test cases for partial writes and invalid indices.

Fixes: https://github.com/nodejs/node/issues/8724

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

src, buffer