wgengine/magicsock: make receive from didCopy respect cancellation.

Very rarely, cancellation occurs between a successful send on derpRecvCh
and a call to copyBuf on the receiving side.
Without this patch, this situation results in <-copyBuf blocking indefinitely.

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -1160,7 +1160,15 @@ func (c *Conn) runDerpReader(ctx context.Context, derpFakeAddr netaddr.IPPort, d
 		case <-ctx.Done():
 			return
 		case c.derpRecvCh <- res:
-			<-didCopy
+			continue
+		}
+		// The copy will not happen if connCtx is cancelled before we reach copyBuf.
+		// This has resulted in a rare inifite wait in practice.
+		select {
+		case <-ctx.Done():
+			return
+		case <-didCopy:
+			continue
 		}
 	}
 }