alloc: Fix UBSAN for accessing the array at -16

This test will check that we exhaust the available TFIs, so the last
TFI is -16 and then we try to store to that address. It is surprising
that it worked!
diff --git a/tests/alloc/AllocTest.cpp b/tests/alloc/AllocTest.cpp
index c3efd4d..b636b6e 100644
--- a/tests/alloc/AllocTest.cpp
+++ b/tests/alloc/AllocTest.cpp
@@ -150,9 +150,9 @@
 		if (tbfs[i])
 			tbf_free(tbfs[i]);
 
-	tbfs[tfi] = tbf_alloc(bts, NULL, dir, -1, 0, 0, 0);
-	OSMO_ASSERT(tbfs[tfi]);
-	tbf_free(tbfs[tfi]);
+	tbfs[0] = tbf_alloc(bts, NULL, dir, -1, 0, 0, 0);
+	OSMO_ASSERT(tbfs[0]);
+	tbf_free(tbfs[0]);
 }
 
 static void test_alloc_a()