rand.Int() would panic when the max value is <= 0, which happens when
big.NewInt() was fed with a too large number. Avoid this by validating
the big.NewInt() return beforehand. Add error handling to all callers to
both gracefully return to IRC and to log an error message.
Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>
