Don't compare pointers with 0, use NULL when needed.
Don't assign pointer to 0, use NULL.
Detected through 'sparse' tool.
Change-Id: Ifa81ba961c0d490cc74880b4a46b620e6358f779
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7598
Tested-by: jenkins