target: arm_dpm: with pointers, use NULL instead of 0

Don't assign pointer to 0, use NULL.

Detected through 'sparse' tool.

Change-Id: I34551112ddab9dedf8537c8111d32356c170e7d5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7669
Tested-by: jenkins

src/target/arm_dpm.c

@@ -1050,7 +1050,7 @@ int arm_dpm_setup(struct arm_dpm *dpm)
 {
 	struct arm *arm = dpm->arm;
 	struct target *target = arm->target;
-	struct reg_cache *cache = 0;
+	struct reg_cache *cache = NULL;
 
 	arm->dpm = dpm;