tun: fix "clear src/dst addresses and checksum before sending"

clearing of addresses and checksum was put in wrong place, so it null'd everything before writing to tun interface.
damnit jeff.

llarp/handlers/tun.cpp

@@ -440,12 +440,6 @@ namespace llarp
       // called in the isolated network thread
       TunEndpoint *self = static_cast< TunEndpoint * >(tun->user);
       self->m_NetworkToUserPktQueue.Process([self, tun](net::IPv4Packet &pkt) {
-        // prepare packet for insertion into network
-        pkt.UpdateChecksumsOnSrc();
-        // clear addresses
-        pkt.src(0);
-        pkt.dst(0);
-
         if(!llarp_ev_tun_async_write(tun, pkt.buf, pkt.sz))
           llarp::LogWarn("packet dropped");
       });
@@ -468,8 +462,15 @@ namespace llarp
       llarp::LogDebug("got pkt ", sz, " bytes");
       if(!self->m_UserToNetworkPktQueue.EmplaceIf(
              [self, buf, sz](net::IPv4Packet &pkt) -> bool {
-               return pkt.Load(llarp::InitBuffer(buf, sz))
-                   && pkt.Header()->version == 4;
+               if(!pkt.Load(llarp::InitBuffer(buf, sz))
+                  || pkt.Header()->version != 4)
+                 return false;
+               // prepare packet for insertion into network
+               pkt.UpdateChecksumsOnSrc();
+               // clear addresses
+               pkt.src(0);
+               pkt.dst(0);
+               return true;
              }))
       {
         llarp::LogInfo("Failed to parse ipv4 packet");