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), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
src, buffer