openocd: remove last NULL comparisons

openocd: remove last NULL comparisons

The NULL pointers preceded by cast where not detected by the
scripting tools looking for NULL pointer comparison.

Remove them and, while there, further simplify the code and apply
the other coding style rules.

Change-Id: Ia7406122e07ef56ef311579ab0ee7ddb22c8e4b5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6539
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>