When zone update is disabled, dns_diff_clear() could be called in setserial() on an uninitialized variable.
Fortunately, in the only place where setserial() is being called, that exact condition is being checked against, so in practice, this scenario should not happen in the current code.
Nonetheless, fix the issue by initializing
diff a little bit
earlier in the function's body.
Closes #3155 (closed)