Use NULL where NULL is meant

git-svn-id: http://liblouis.googlecode.com/svn/trunk@1157 24698d54-e344-0410-af24-25bd253c2c7b
diff --git a/liblouis/compileTranslationTable.c b/liblouis/compileTranslationTable.c
index c55c176..dbe2836 100644
--- a/liblouis/compileTranslationTable.c
+++ b/liblouis/compileTranslationTable.c
@@ -5171,7 +5171,7 @@
 static logcallback logCallbackFunction = defaultLogCallback;
 void EXPORT_CALL lou_registerLogCallback(logcallback callback)
 {
-  if (callback == 0)
+  if (callback == NULL)
     logCallbackFunction = defaultLogCallback;
   else
     logCallbackFunction = callback;
@@ -5189,7 +5189,7 @@
       return;
   if (level < logLevel)
       return;
-  if (logCallbackFunction != 0)
+  if (logCallbackFunction != NULL)
     {
       char *s;
       size_t len;