Fix APR crashes in read rssi callback

When read RSSI timeouts, callback is called with NULL result. This was
causing crash.

Bug: 30562545
Change-Id: If379c2de4ceab48c021601851a8c83f995440460
diff --git a/btif/src/btif_gatt_client.c b/btif/src/btif_gatt_client.c
index 57b5958..c70fa49 100644
--- a/btif/src/btif_gatt_client.c
+++ b/btif/src/btif_gatt_client.c
@@ -1023,6 +1023,9 @@
 
 static void btm_read_rssi_cb (tBTM_RSSI_RESULTS *p_result)
 {
+    if (!p_result)
+      return;
+
     btif_gattc_cb_t btif_cb;
 
     bdcpy(btif_cb.bd_addr.address, p_result->rem_bda);