bootstd: rauc: Fix null pointer access while checking root part

bootstd: rauc: Fix null pointer access while checking root part

Fix a segmentation fault caused by a null pointer access during root
partition checking. The function part_get_info() was falsely given null
for the disk_partition struct, which later resulted in accessing a null
pointer and thus undefined behavior.

Fixes: 5d7c080ae5dc ("bootstd: rauc: Don't check root part filesystem")
Signed-off-by: Martin Schwan <m.schwan@phytec.de>