tests: Fix the sms_test and use the right array

It started to behave weird on Debian Testing (GCC 4.8), I compiled it
with address sanitizer support and set a breakpoint in
__asan_report_error to get a backtrace.
diff --git a/tests/sms/sms_test.c b/tests/sms/sms_test.c
index a79d454..8e10241 100644
--- a/tests/sms/sms_test.c
+++ b/tests/sms/sms_test.c
@@ -371,7 +371,7 @@
 		memset(result, 0xaa, sizeof(result));
 
 		for (buffer_size = 1;
-		     buffer_size < test_encode[i].expected_septet_length + 1
+		     buffer_size < test_decode[i].expected_septet_length + 1
 		     && buffer_size < sizeof(result) - 1;
 		     ++buffer_size)
 		{