arm: cortex-m: initialise ptr_esf in get_esf() in fault.c.
arm: cortex-m: initialise ptr_esf in get_esf() in fault.c.

This was producing a -Wsometimes-uninitialized warning.

Signed-off-by: Chris Reed <5c8e7a799cb891a0ec7887f2dd5d3bfb138b5f9c@arm.com>

zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr