Failed to request an autovacuum work-item in silence

Hi all,

While reading the code, I realized that the requesting an autovacuum
work-item could fail in silence if work-item array is full. So the
users cannot realize that work-item is never performed.
AutoVacuumRequestWork() seems to behave so from the initial
implementation but is there any reason of such behavior? It seems to
me that it can be a problem even now that there is only one kind of
work-item. Attached patch for fixing it.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center