make this more resilient to stream problems

The metafile stream read in ooxmlexport test for fdo51550.odt is broken
for some reason, so mnTotalSize is not initialized. That means that we
try to seek by an arbitrary value in the destructor.

Change-Id: I3bd5ef3708c816fc8a749b446cecf98c3aa52f65

tools/source/stream/vcompat.cxx

@@ -22,6 +22,8 @@
 
 VersionCompat::VersionCompat( SvStream& rStm, sal_uInt16 nStreamMode, sal_uInt16 nVersion ) :
             mpRWStm     ( &rStm ),
+            mnCompatPos ( 0 ),
+            mnTotalSize ( 0 ),
             mnStmMode   ( nStreamMode ),
             mnVersion   ( nVersion )
 {