fs: don't throw in read if buffer too big

If the resulting buffer.toString() call in fs.read throws, catch the
error and pass it back in the callback.

This issue only presents itself when fs.read is called using the legacy
string interface:

fs.read(fd, length, position, encoding, callback)

PR-URL: https://github.com/nodejs/node/pull/3503


Reviewed-By: Trevor Norris <trev.norris@gmail.com>