Fix "error: Forever detected script exited with code: null"

[hughescr]

When a process dies from a signal and has no exit code, forever logs this incorrectly. It is checking isNaN(code), which is wrong. isNaN(x) does not check that x is numeric; it merely checks if x is equal to NaN; since code is either null or an integer, this can never be true in this context. isNaN(null) === false.

This patch corrects this bad check, to verify that code was in fact passed by the exit handler. If it wasn't, then we fall through to print the signal that killed the monitored process.

[ericdolson]

I would love to see this fix added. I am having an issue where I am trying to SIGKILL my app programmatically which results in a code===null. But since the isNan(null) resolves to true, my app restarts when it should not.

[emaV]

This solve my issue with upstart. When trying to stop the service the app return null and forever keeps restarting when it should simply stop.

[michaelmcmillan]

Should this not be merged?

[indexzero]

This looks good at first glance, but we should have test coverage for this before merging it in.