Currently, if jail_getid(3) is passed in a numeric string, it assumes that this is a jid string and passes it back converted to an int without checking that it's a valid/existing jid. This breaks consumers that might use jail_getid(3) to see if it can trivially grab a jid from a name if that name happens to be numeric but not actually the name/jid of the jail. Instead of returning -1 for the jail not existing, it'll return the int version of the input and the consumer will not fallback to trying other methods.
Pass the numeric input to jail_get(2) as the jid for validation, rather than the name.