serial: nrfx_uarte: check size before writing to buffer
serial: nrfx_uarte: check size before writing to buffer

NRFX UARTE would write to user supplied buffer on IRQ without checking
whether or not the supplied buffer had available space left to write
one char

Signed-off-by: Arne Edholm <c7713a92ea54b3f486ddd0db13cac5f6fb76e03c@assaabloy.com>

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