src/bootchooser: r_boot_get_state: must not be called for non-boot slot
src/bootchooser: r_boot_get_state: must not be called for non-boot slot

Add this sanity check to detect programatic errors where this funtion is
called for a slot that is not a boot slot.
Trying to get the boot state for a slot that is not a boot slot is
simply wrong.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,rauc/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,jluebbe/rauc