Bug 950891: Don't run GetServiceChannel while turning Bluetooth on/off, r=echou

This patch adds a check to BluetoothDBusService::GetServiceChannel to
ensure that Bluetooth is ready. If the system is in a transition from
on to off, or vice versa, the method might operate on an undefined
state.

dom/bluetooth/bluez/linux/BluetoothDBusService.cpp

@@ -2886,6 +2886,11 @@ BluetoothDBusService::GetServiceChannel(const nsAString& aDeviceAddress,
 {
   MOZ_ASSERT(NS_IsMainThread());
 
+  if (!IsReady()) {
+    NS_NAMED_LITERAL_STRING(errorStr, "Bluetooth service is not ready yet!");
+    return NS_OK;
+  }
+
   MOZ_ASSERT(!sAdapterPath.IsEmpty());
   nsString objectPath(GetObjectPathFromAddress(sAdapterPath, aDeviceAddress));