Fix: fmt::ostream cannot be moved while holding buffered data #2197 (#2198)

* Add a test case: move ostream while holding data

* Fix moving ostream while holding data

Co-authored-by: Junliang HU <jlhu@cse.cuhk.edu.hk>

include/fmt/os.h

@@ -396,8 +396,10 @@ class ostream final : private detail::buffer<char> {
   ostream(ostream&& other)
       : detail::buffer<char>(other.data(), other.size(), other.capacity()),
         file_(std::move(other.file_)) {
+    other.clear();
     other.set(nullptr, 0);
   }
+
   ~ostream() {
     flush();
     delete[] data();