Fix segfault in cortex_m_deinit_target

Introduced by f92d7174cc "target/arm_adi_v5: power off chip debug
hardware on shutdown".

Change-Id: Ifc29295eaf82cb73347dfb578f878896699ea959

src/target/cortex_m.c

@@ -1800,7 +1800,8 @@ void cortex_m_deinit_target(struct target *target)
 {
 	struct cortex_m_common *cortex_m = target_to_cm(target);
 
-	dap_dp_uninit(cortex_m->armv7m.debug_ap->dap);
+        if (cortex_m->armv7m.debug_ap)
+                dap_dp_uninit(cortex_m->armv7m.debug_ap->dap);
 
 	free(cortex_m->fp_comparator_list);