Initialize gen_ variable

Change-Id: I87e0b2e11670ea71da8e4d2f718dc97dd32dd57d
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/tools/qdoc/qdocindexfiles.cpp

@@ -64,6 +64,7 @@ QDocIndexFiles* QDocIndexFiles::qdocIndexFiles_ = NULL;
   Constructs the singleton QDocIndexFiles.
  */
 QDocIndexFiles::QDocIndexFiles()
+    : gen_( 0 )
 {
     qdb_ = QDocDatabase::qdocDB();
 }