vcl: fix incorrect ldp worker in ldp_epoll_pwait()

vcl: fix incorrect ldp worker in ldp_epoll_pwait()

For some apps(e.g. wrk2) upon vpp hoststack, ldp_epoll_pwait()
is called. In this function, epoll fd was created on one thread,
but it is now used on another thread. The vcl worker index is still
invalid, so the fetched ldp worker is also invalid and can corrupt
some already allocated memory.

Just as the ldp_epoll_pwait_eventfd(), make sure the vcl worker is valid
before getting the ldp worker in ldp_epoll_pwait().

Type: fix

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: I2ec23a4b5d5b0879a06642ffd80f95e948af4274