When we shut down, we destroy the various caches, in the process of
which SalBitmap calls back into it's owning cache, causing a SIGSEGV.

Found while loading files from tdf#83426

Change-Id: I53db1621a0fdb75a8e66582662b0e2666499192b
Reviewed-on: https://gerrit.libreoffice.org/73387
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>