Fix nullptr in btif_hf_client

When cb is nullptr, this can cause a crash in lines below.

Test: manual
Bug: 180420059
Tag: #security
Change-Id: I09156852f6a99315bf4aebf948efee21a7bcfa1c
diff --git a/btif/src/btif_hf_client.cc b/btif/src/btif_hf_client.cc
index 3a8e4ea..6dc81ce 100644
--- a/btif/src/btif_hf_client.cc
+++ b/btif/src/btif_hf_client.cc
@@ -164,6 +164,7 @@
   btif_hf_client_cb_t* cb = btif_hf_client_get_cb_by_bda(*bd_addr);
   if (cb == NULL || !is_connected(cb)) {
     BTIF_TRACE_ERROR("%s: failed to find block for bda", __func__);
+    return;
   }
 
   BTIF_TRACE_EVENT("%s: event=%d", __func__, event);