[findBugs] Fix potential NPE in GC

Change-Id: I59cda76b2c5039e08612f394ee4f7f1788578c49
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java

@@ -1204,9 +1204,10 @@ public class GC {
 				new DirectoryStream.Filter<Path>() {
 
 					public boolean accept(Path file) throws IOException {
-						return Files.isRegularFile(file) && PATTERN_LOOSE_OBJECT
+						Path fileName = file.getFileName();
-								.matcher(file.getFileName().toString())
+						return Files.isRegularFile(file) && fileName != null
-								.matches();
+								&& PATTERN_LOOSE_OBJECT
+										.matcher(fileName.toString()).matches();
 					}
 				})) {
 			for (Iterator<Path> iter = stream.iterator(); iter.hasNext();