flash/stm32l4x: fix segmentation fault with HLA adapters and STM32WLx devices

flash/stm32l4x: fix segmentation fault with HLA adapters and STM32WLx devices

CPU2 (Cortex-M0+) is supported only with non-hla adapters because it is on AP1.
Using HLA adapters armv7m.debug_ap is null, and checking ap_num triggers
a segfault.

Change-Id: I501f5b69e629aa8d2836b5194063d74d5bfddb12
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Fixes: https://github.com/STMicroelectronics/OpenOCD/issues/6
Reviewed-on: https://review.openocd.org/c/openocd/+/6535
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>