valgrind: fix leak

since...

commit e94d5233
Date:   Fri Mar 11 06:43:06 2016 +0100

    work on sane lifecylce for SfxFilter

which removed "delete pFilter" though it was
pMediumFilter which was in question in this method

Change-Id: I598dd44a8498ebd6b3e63d1c89147bee5ab3bb55

sd/source/ui/docshell/docshel4.cxx

@@ -642,6 +642,8 @@ bool DrawDocShell::ConvertTo( SfxMedium& rMedium )
             bRet = pFilter->Export();
             if( !bRet )
                 mpDoc->SetSwapGraphicsMode( nOldSwapMode );
+
+            delete pFilter;
         }
     }