Fix js.exec() returned nul" unless exit() was called explicitly

Don't use the "exit_code" property value as the return value of js.exec() unless it's a number. As reported by mlong (thanks).

Also, "exit_code" was being set to null (instead of void/undefined) in js_PrepareToExecute(). I think this was just an oversight or typo by Deuce from his commit of 5 years ago (f3256d81). Since we're comparing exit_code with JSVAL_VOID in other places to determine if it was actually set, this appears to be a long standing bug.