arm_adi_v5: fix scan-build warning [3/3]

arm_adi_v5: fix scan-build warning [3/3]

While scan-build complains that dap_p or ap_num_p could be NULL,
the current code never passes NULL pointers.
Add an assert() to silent scan-build and prevent any further use
of the function with incorrect parameters.

Change-Id: I656810dddcea61e85d85b13efb114f7607ef837c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7012
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>