ypldap(8): Fix absent va_end() in 'goto fail' path

In this path, we used va_start() without pairing it with va_end(). Add the
va_end(). (va_start() without paired va_end() is undefined behavior per the C
standard.)

"In many implementations, [va_end] is a do-nothing operation; but those
implementations that need it probably need it badly." - Rationale for the ANSI
C Programming Language, § 4.8.1.3.

Reported by: Coverity
CID: 1340539