Remove extra conditional from FakeStorageRpc#list

[athakor]

Remaining clean up from PR #208
From: @frankyn
File: https://github.com/googleapis/java-storage-nio/blob/master/google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/testing/FakeStorageRpc.java

Saw this after, but it can help to modify the first conditional, to short circuit sooner in the sequence of code.

if (!so.getBucket().equals(bucket) || !so.getName().startsWith(prefix)) {
        continue;
}