Robustify PropItem::Read.

Initialize nType and nItemSize variables in case stream is faulty.

Change-Id: I59602f2b04e3816d8de7cc1a5e7e526c8e822477
Reviewed-on: https://gerrit.libreoffice.org/4762Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>

sd/source/filter/ppt/propread.cxx

@@ -82,10 +82,14 @@ sal_Bool PropItem::Read( OUString& rString, sal_uInt32 nStringType, sal_Bool bAl
     nItemPos = Tell();
 
     if ( nStringType == VT_EMPTY )
+    {
+        nType = VT_NULL; // Initialize in case stream fails.
         *this >> nType;
+    }
     else
         nType = nStringType & VT_TYPEMASK;
 
+    nItemSize = 0; // Initialize in case stream fails.
     *this >> nItemSize;
 
     switch( nType )