src: fix buffer overflow for long exception lines

Long exception lines resulted in a stack buffer overflow or assertion
because it was assumed snprintf not counts discarded chars
or the assertion itself was incorrect: `(off) >= sizeof(arrow)`

PR-URL: https://github.com/nodejs/node/pull/2404


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>