libpdx_uds: Fix uninitialized variable bug.

In this code path epoll_wait() returns 0 when the operation times
out, but there is no condition to catch this and return the
approptiate status. This allows the event value to be used
uninitialized, which returns a junk pending event mask.

This bug results in BufferHub queues sometimes thinking that the
producer side of the buffer hung up, when the junk value happens
to have the EPOLLHUP bit set.

Bug: 62886596
Test: Observe VR app switching stability.
Change-Id: Id9554d6bf224fd27815ff042ac145f59041d0aae