Some blkmap slices (like blkmap_crypt) don't support writes and have
their write function set to NULL. The blkmap_blk_write_slice() function
calls the write function without checking if it's NULL, causing a crash
when attempting to write to such slices.
Add a NULL check before calling the write function. When the slice
doesn't support writes, return 0 to indicate no blocks were written.
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
8ff7005997