Adds a fuzz target (#908)

* Adds a fuzz target

* Fixes memory leak

_example/fuzz/fuzz_openexec.go

@@ -0,0 +1,30 @@
+package sqlite3_fuzz
+
+import (
+	"bytes"
+	"database/sql"
+	"io/ioutil"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+func FuzzOpenExec(data []byte) int {
+	sep := bytes.IndexByte(data, 0)
+	if sep <= 0 {
+		return 0
+	}
+	err := ioutil.WriteFile("/tmp/fuzz.db", data[sep+1:], 0644)
+	if err != nil {
+		return 0
+	}
+	db, err := sql.Open("sqlite3", "/tmp/fuzz.db")
+	if err != nil {
+		return 0
+	}
+	defer db.Close()
+	_, err = db.Exec(string(data[:sep-1]))
+	if err != nil {
+		return 0
+	}
+	return 1
+}

sqlite3.go

@@ -1676,7 +1676,7 @@ func (d *SQLiteDriver) Open(dsn string) (driver.Conn, error) {
 	//
 	// Because default is NORMAL this statement is always executed
 	if err := exec(fmt.Sprintf("PRAGMA synchronous = %s;", synchronousMode)); err != nil {
-		C.sqlite3_close_v2(db)
+		conn.Close()
 		return nil, err
 	}