fs: fat: avoid NULL dereference when root dir is full

When trying to create a file in the full root directory of a FAT32
filesystem a NULL dereference can be observed.

When the root directory of a FAT16 filesystem is full fill_dir_slot() must
return -1 to signal that a new directory entry could not be allocated.

Fixes: cd2d727f ("fs: fat: allocate a new cluster for root directory of fat32")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>