V595 The 'GetMedium()' pointer was utilized before it was verified against nullptr.
     Check lines: 435, 455.

bURLChanged is only meaningful when GetMedium() is not nullptr;
otherwise bSaveDS could become true where no medium existed,
and dereferencing GetMedium() to find its URL would segfault.

Change-Id: I6f533acb3b1ca078803aa23bdece0ba2ee1d87c3
Reviewed-on: https://gerrit.libreoffice.org/71467
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>