sw: initialize SwDoc::mbInReading

This was causing intermittent failure at least in sw_python test.
(regression from 5494954b)

Change-Id: I8452536cfbb7bc5bd07e6405bc255e31a5007185

sw/source/core/doc/docnew.cxx

@@ -260,6 +260,7 @@ SwDoc::SwDoc()
     mReferenceCount(0),
     mbGlossDoc(false),
     mbDtor(false),
+    mbInReading(false),
     mbInXMLImport(false),
     mbUpdateTOX(false),
     mbInLoadAsynchron(false),