Fix move constructor (#1844)

include/fmt/os.h

@@ -396,7 +396,9 @@ class ostream : private detail::buffer<char> {
   }
 
  public:
-  ostream(ostream&& other) : file_(std::move(other.file_)) {
+  ostream(ostream&& other)
+      : detail::buffer<char>(other.data(), other.size(), other.capacity()),
+        file_(std::move(other.file_)) {
     other.set(nullptr, 0);
   }
   ~ostream() {

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);