Fix NPE in getMdcCopy of LoggingEventInstrumentation

Author: ygree

dd-java-agent/instrumentation/log4j1/src/main/java/datadog/trace/instrumentation/log4j1/LoggingEventInstrumentation.java

@@ -134,6 +134,10 @@ public static void onExit(
       if (!injectionRequired) {
         return;
       }
+      if (mdc == null) {
+        // this.mdcCopy can be null when MDC.getContext() returns null
+        return;
+      }
       // at this point the mdc has been shallow copied. No need to replace with a new hashtable.
       // Just add our info
       String serviceName = Config.get().getServiceName();

dd-java-agent/instrumentation/log4j1/src/test/groovy/MdcTest.groovy

@@ -16,8 +16,16 @@ class MdcTest extends AgentTestRunner {
     then:
     assert event.getMDC("data") == "dog"
     where:
-    injectionEnabled | _
-    "true"           | _
-    "false"          | _
+    injectionEnabled << ["true", "false"]
+  }
+
+  def "should prevent NPE when MDC context doesn't exist and getMDCCopy"() {
+    setup:
+    injectSysConfig("logs.injection", "true")
+    def event = new LoggingEvent("test", Category.getRoot(), Priority.INFO, "hello world", null)
+    when:
+    event.getMDCCopy()
+    then:
+    noExceptionThrown()
   }
 }