From coverity: listFiles may return null

git-svn-id: http://svn.code.sf.net/p/davmail/code/trunk@2279 3d1905a2-6b24-0410-a738-b14d5a86fcbd

src/java/davmail/exchange/ExchangeSession.java

@@ -2591,11 +2591,13 @@ public abstract class ExchangeSession {
                                 return false;
                             }
                         });
+                        if (oldestFiles != null) {
                             for (File file : oldestFiles) {
                                 if (!file.delete()) {
                                     LOGGER.warn("Unable to delete " + file.getAbsolutePath());
                                 }
                             }
+                        }
                     } catch (Exception ex) {
                         LOGGER.warn("Error deleting ics dump: " + ex.getMessage());
                     }