[spotbugs] Fix potential NPE in OpenSshServerKeyDatabase

If oldLine is null #updateModifiedServerKey shouldn't be called since it
would derefence it. Spotbugs raised this as problem
RCN_REDUNDANT_NULLCHECK_WOULD_HAVE_BEEN_A_NPE. Fix it by checking if
oldLine is null before calling #updateModifiedServerKey.

Change-Id: I8a2000492986e52ce7dbe25f48b321c05fd371e4
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.ssh.apache/src/org/eclipse/jgit/internal/transport/sshd/OpenSshServerKeyDatabase.java

@@ -346,11 +346,14 @@ public class OpenSshServerKeyDatabase
 			throws IOException {
 		KnownHostEntry hostEntry = entry.getHostEntry();
 		String oldLine = hostEntry.getConfigLine();
+		if (oldLine == null) {
+			return;
+		}
 		String newLine = updateHostKeyLine(oldLine, serverKey);
 		if (newLine == null || newLine.isEmpty()) {
 			return;
 		}
-		if (oldLine == null || oldLine.isEmpty() || newLine.equals(oldLine)) {
+		if (oldLine.isEmpty() || newLine.equals(oldLine)) {
 			// Shouldn't happen.
 			return;
 		}