Disallow zero EventLoop threads (#1230)
Disallow zero EventLoop threads (#1230)

Motivation:

Zero EventLoop threads makes no sense and leads to a crash (see #1223)

Modifications:

Add a precondition

Result:

Initializing EventLoop with zero threads will fail precondition rather than crash later.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio