[findBugs] Prevent potential NPE in FS_POSIX.readUmask()

BufferedReader.readLine() returns null if the end of the stream has been
reached.

Change-Id: I83102bbfb1316407247e0f29023077af9e8d9606
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/util/FS_POSIX.java

@@ -122,7 +122,7 @@ public class FS_POSIX extends FS {
 							.defaultCharset().name()))) {
 				if (p.waitFor() == 0) {
 					String s = lineRead.readLine();
-					if (s.matches("0?\\d{3}")) { //$NON-NLS-1$
+					if (s != null && s.matches("0?\\d{3}")) { //$NON-NLS-1$
 						return Integer.parseInt(s, 8);
 					}
 				}