lib/rbtree: Fix crash condition with empty trees and rb_min/max()
lib/rbtree: Fix crash condition with empty trees and rb_min/max()

These weren't properly checking the case of an empty tree

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

galak/zephyr,punitvara/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyriot/zephyr,ldts/zephyr,zephyriot/zephyr,mbolivar/zephyr,kraj/zephyr,zephyriot/zephyr,mbolivar/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,finikorg/zephyr,nashif/zephyr,kraj/zephyr,ldts/zephyr,mbolivar/zephyr,explora26/zephyr,kraj/zephyr,ldts/zephyr,zephyriot/zephyr,punitvara/zephyr,Vudentz/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,Vudentz/zephyr,galak/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,finikorg/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,mbolivar/zephyr