Fix a race between resolver query timeout and validation
The resolver.c:validated() function unlinks the current validator from
the fetch's validators list, which can leave it empty, then unlocks
the bucket lock. If, by a chance, the fetch was timed out just before
the validated() call, the final timeout callback running in parallel
with validated() can find the fetch context with no active fetches
and with an empty validators list and destroy it, which is unexpected
for the validated() function and can lead to a crash.
Increase the fetch context's reference count in the beginning of
validated() and decrease it when it finishes its work to avoid the
unexpected destruction of the fetch context.
Closes #3398 (closed)