i#64348 basic: fix CDec() crash if string is empty

Change-Id: I92e9472e14c00a6550081f0d58a352faa5b78b98
Reviewed-on: https://gerrit.libreoffice.org/9337
Tested-by: David Tardon <dtardon@redhat.com>
Reviewed-by: David Tardon <dtardon@redhat.com>