Fix data race introduced on 605d9d08

This commit will fix a data race introduced on 605d9d08 and also
make sure travis CI will now check for data races.

.travis.yml

@@ -10,4 +10,4 @@ before_install:
   - go get golang.org/x/tools/cmd/cover
 script:
   - $HOME/gopath/bin/goveralls -repotoken 3qJVUE0iQwqnCbmNcDsjYu1nh4J4KIFXx
-  - go test -v . -tags "libsqlite3"
+  - go test -race -v . -tags "libsqlite3"

sqlite3.go

@@ -847,7 +847,6 @@ func (rc *SQLiteRows) Close() error {
 	}
 	if rc.done != nil {
 		close(rc.done)
-		rc.done = nil
 	}
 	if rc.cls {
 		return rc.s.Close()