Do not assume sqlite3_column_text is valid UTF-8.

Fix Statement::value_ref

src/pragma.rs

@@ -86,6 +86,7 @@ impl Sql {
                 self.push_real(r);
             }
             ValueRef::Text(s) => {
+                let s = std::str::from_utf8(s).expect("invalid UTF-8");
                 self.push_string_literal(s);
             }
             _ => {