goldfish: avoid crashing netmgr

When host side does not provide network forwarding service

BUG: 119328108
Change-Id: If69eebd401bc25f2de69127164b9cf72545d7915
diff --git a/network/netmgr/wifi_forwarder.cpp b/network/netmgr/wifi_forwarder.cpp
index 64f5b66..e6997d4 100644
--- a/network/netmgr/wifi_forwarder.cpp
+++ b/network/netmgr/wifi_forwarder.cpp
@@ -185,6 +185,9 @@
 
 
 void WifiForwarder::getPollData(std::vector<pollfd>* fds) const {
+    if (mPipeFd == -1) {
+        return;
+    }
     int pcapFd = pcap_get_selectable_fd(mMonitorPcap);
     if (pcapFd != -1) {
         fds->push_back(pollfd{pcapFd, POLLIN, 0});