s390/test_unwind: fix possible memleak in test_unwind()

test_unwind() misses to call kfree(bt) in an error path.
Add the missed function call to fix it.

Fixes: 0610154650f1 ("s390/test_unwind: print verbose unwinding results")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Wang Hai <[e-mail]>
Acked-by: Ilya Leoshkevich <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>

arch/s390/lib/test_unwind.c

···
 			break;
 		if (state.reliable && !addr) {
 			pr_err("unwind state reliable but addr is 0\n");
+			kfree(bt);
 			return -EINVAL;
 		}
 		sprint_symbol(sym, addr);