tests: ms: Pass correct pointer in constructor instead of NULL

The BTS field will be used in code paths after next patch changes,
otherwise the test fails accessing the NULL pointer.

Change-Id: I5098292bdafa9f4f70fef1a053b80a33deca722c

tests/ms/MsTest.cpp

@@ -369,7 +369,7 @@ static void test_ms_storage()
 	gprs_rlcmac_ul_tbf *ul_tbf;
 	BTS the_bts;
 	GprsMs *ms, *ms_tmp;
-	GprsMsStorage store(NULL);
+	GprsMsStorage store(&the_bts);
 
 	printf("=== start %s ===\n", __func__);