SQLite plugin: use QString::unicode(), not utf16()

utf16() reallocates a QString if it was created from raw data, in order
to ensure NUL termination. But here we don't need NUL termination
anyways because we also pass the string size, so just use unicode()
instead.

Change-Id: I4a01ab9f4e53b94b80d3d00272cb0f0e35e30959
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/plugins/sqldrivers/sqlite/qsql_sqlite.cpp

@@ -540,7 +540,7 @@ bool QSQLiteResult::exec()
                 case QMetaType::QString: {
                     // lifetime of string == lifetime of its qvariant
                     const QString *str = static_cast<const QString*>(value.constData());
-                    res = sqlite3_bind_text16(d->stmt, i + 1, str->utf16(),
+                    res = sqlite3_bind_text16(d->stmt, i + 1, str->unicode(),
                                               int(str->size()) * sizeof(QChar),
                                               SQLITE_STATIC);
                     break; }