target: armv8: Avoid semihosting segfault on halt

Avoid a NULL pointer dereference when halting an aarch64 core.

Change-Id: I333d40475ab26e2f0dca5c27302a5fa4d817a12f
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/4593
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/target/armv8.c

@@ -1063,7 +1063,7 @@ int armv8_aarch64_state(struct target *target)
 		armv8_mode_name(arm->core_mode),
 		buf_get_u32(arm->cpsr->value, 0, 32),
 		buf_get_u64(arm->pc->value, 0, 64),
-		target->semihosting->is_active ? ", semihosting" : "");
+		(target->semihosting && target->semihosting->is_active) ? ", semihosting" : "");
 
 	return ERROR_OK;
 }