Improve error message "mdb.c(319): non-null pointer"

Per Support ticket RT #14122

In a dhcpd.conf that contains both client identifier AND uid in the same host declaration, the following warning message is emitted as dhcpd starts:

mdb.c(319): non-null pointer

Having both is ambiguous and not supported - but the error message is not in the least helpful or useful for diagnosing what is wrong.