#646 Add volatile where required for double checked locking

Without volatile we can have the situation where thread 2 sees the value of initPending to be false and thus doesn't go through the synced section but sees loggers as not yet visible.

This was rare and the test only failed on the first run on my machine before always passing.

Hopefully, it should always work now. Fingers crossed.

NOTE: Both of the changed classes use a version of the "double checked locking" pattern which the internet says requires volatile.

openhtmltopdf-core/src/main/java/com/openhtmltopdf/util/JDKXRLogger.java

@@ -35,11 +35,11 @@
  * https://github.com/danfickle/openhtmltopdf/wiki/Logging
  */
 public class JDKXRLogger implements XRLogger {
-    private boolean initPending = true;
+    private volatile boolean initPending = true;
 
     // Keep a map of Loggers so they are not garbage collected
     // which makes them lose their settings we have applied.
-    private Map<String, Logger> loggers;
+    private volatile Map<String, Logger> loggers;
 
     private final boolean useParent;
     private final Level level;

openhtmltopdf-core/src/main/java/com/openhtmltopdf/util/XRLog.java

@@ -52,8 +52,8 @@ private static String registerLoggerByName(final String loggerName) {
         return loggerName;
     }
 
-    private static boolean initPending = true;
-    private static XRLogger loggerImpl;
+    private static volatile boolean initPending = true;
+    private static volatile XRLogger loggerImpl;
 
     private static volatile Boolean loggingEnabled;