JDK 9 RFR of 8056205: (fs) Potential for NPE in Files.walkFileTree if closing directory fails

[Brian Burkhalter]

Please review this simple fix at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8056205
Patch:	http://cr.openjdk.java.net/~bpb/8056205/webrev.00/

Correct an obviously erroneous block of the form:

	if (x != null) {
		x = y;
	} else {
		x.someMethod(y); // guaranteed NPE
	}

Thanks,

Brian