target: fix null pointer exception

check if no target is selected and return error.

Change-Id: Ie8abb63c708d09572b45e88fc6766af108715077
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/148
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/target/armv4_5.c

@@ -741,7 +741,13 @@ COMMAND_HANDLER(handle_arm_disassemble_command)
 {
 	int retval = ERROR_OK;
 	struct target *target = get_current_target(CMD_CTX);
-	struct arm *arm = target ? target_to_arm(target) : NULL;
+
+	if (target == NULL) {
+		LOG_ERROR("No target selected");
+		return ERROR_FAIL;
+	}
+
+	struct arm *arm = target_to_arm(target);
 	uint32_t address;
 	int count = 1;
 	int thumb = 0;
@@ -946,7 +952,13 @@ static int jim_mcrmrc(Jim_Interp *interp, int argc, Jim_Obj *const *argv)
 COMMAND_HANDLER(handle_arm_semihosting_command)
 {
 	struct target *target = get_current_target(CMD_CTX);
-	struct arm *arm = target ? target_to_arm(target) : NULL;
+
+	if (target == NULL) {
+		LOG_ERROR("No target selected");
+		return ERROR_FAIL;
+	}
+
+	struct arm *arm = target_to_arm(target);
 
 	if (!is_arm(arm)) {
 		command_print(CMD_CTX, "current target isn't an ARM");