Fix move constructor (#1844)

test/os-test.cc

@@ -287,6 +287,12 @@ TEST(BufferedFileTest, Fileno) {
   EXPECT_READ(copy, FILE_CONTENT);
 }
 
+TEST(OStreamTest, Move) {
+  fmt::ostream out = fmt::output_file("test-file");
+  fmt::ostream moved(std::move(out));
+  moved.print("hello");
+}
+
 TEST(OStreamTest, Print) {
   fmt::ostream out = fmt::output_file("test-file");
   out.print("The answer is {}.\n", 42);