CVE-2024-42235 s390/mm: Add NULL pointer check to crst_table_free() base_crst_free()

In the Linux kernel, the following vulnerability has been resolved:

s390/mm: Add NULL pointer check to crst_table_free() base_crst_free()

crst_table_free() used to work with NULL pointers before the conversion
to ptdescs. Since crst_table_free() can be called with a NULL pointer
(error handling in crst_table_upgrade() add an explicit check.

Also add the same check to base_crst_free() for consistency reasons.

In real life this should not happen, since order two GFP_KERNEL
allocations will not fail, unless FAIL_PAGE_ALLOC is enabled and used.