check for missing result of rate_ctr_group_alloc()

In case the counter group allocation fails, we must handle this
gracefully and fail the allocation of the parent object, too.

Change-Id: Id6d780c67b4af15aaa5c6f2b8b00f2a0b70a7385
Related: OS#2361
diff --git a/src/bts.cpp b/src/bts.cpp
index 5192646..2d289d7 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -207,7 +207,9 @@
 	}
 
 	m_ratectrs = rate_ctr_group_alloc(tall_pcu_ctx, &bts_ctrg_desc, 0);
+	OSMO_ASSERT(m_ratectrs);
 	m_statg = osmo_stat_item_group_alloc(tall_pcu_ctx, &bts_statg_desc, 0);
+	OSMO_ASSERT(m_statg);
 }
 
 BTS::~BTS()