buffer: add a check for byteLength in readIntBE() and readIntLE()

The 'byteLength' argument should be required and of type 'number'. It should have a value between 1 and 6.

This is a fix for issue: https://github.com/nodejs/node/issues/10515

Checklist

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

Affected core subsystem(s)

buffer