CppunitTest_sw_htmlexport: check for nullptr

I couldn't reproduce this one, but JBFaure says on IRC it helps in his
case.

Change-Id: Ib0a22996bdfc758326c4910b5b652f0772c08912

sw/qa/extras/htmlexport/htmlexport.cxx

@@ -38,7 +38,7 @@ private:
 
     void preTest(const char* filename) SAL_OVERRIDE
     {
-        if (OString(filename) == "charborder.odt")
+        if (OString(filename) == "charborder.odt" && SW_MOD())
         {
             // FIXME if padding-top gets exported as inches, not cms, we get rounding errors.
             SwMasterUsrPref* pPref = const_cast<SwMasterUsrPref*>(SW_MOD()->GetUsrPref(false));
@@ -49,7 +49,7 @@ private:
 
     void postTest(const char* filename) SAL_OVERRIDE
     {
-        if (OString(filename) == "charborder.odt")
+        if (OString(filename) == "charborder.odt" && SW_MOD())
         {
             SwMasterUsrPref* pPref = const_cast<SwMasterUsrPref*>(SW_MOD()->GetUsrPref(false));
             pPref->SetMetric(m_eUnit);