Fix an init bug in global.c

* globals.c: fix the initialization of the mutex
diff --git a/globals.c b/globals.c
index 1f807d8..69002f0 100644
--- a/globals.c
+++ b/globals.c
@@ -46,7 +46,7 @@
  */
 void xmlInitGlobals(void)
 {
-    if (xmlThrDefMutex != NULL)
+    if (xmlThrDefMutex == NULL)
         xmlThrDefMutex = xmlNewMutex();
 }