Fix error handling in xml_document::save_file

There were two conditions under which xml_document::save_file could
previously return true even though the saving failed:

- The last write to the file was buffered in stdio buffer, and it's that
  last write that would fail due to lack of disk space
- The data has been written correctly but fclose failed to update file
  metadata, which can result in truncated size / missing inode updates.

This change fixes both by adjusting save_file to fflush before the check,
and also checking fclose results. Note that while fflush here is
technically redundant, because it's implied by fclose, we must check
ferror explicitly anyway, and so it feels a little cleaner to do most of
the error handling in save_file_impl, so that the changes of fclose()
failing are very slim.

Of course, neither change guarantees that the contents of the file are
going to be safe on disk following a power failure.

src/pugixml.cpp

@@ -5070,7 +5070,7 @@ PUGI__NS_BEGIN
 		xml_writer_file writer(file);
 		doc.save(writer, indent, flags, encoding);
 
-		return ferror(file) == 0;
+		return fflush(file) == 0 && ferror(file) == 0;
 	}
 
 	struct name_null_sentry
@@ -7423,15 +7423,15 @@ namespace pugi
 		using impl::auto_deleter; // MSVC7 workaround
 		auto_deleter<FILE> file(impl::open_file(path_, (flags & format_save_file_text) ? "w" : "wb"), impl::close_file);
 
-		return impl::save_file_impl(*this, file.data, indent, flags, encoding);
+		return impl::save_file_impl(*this, file.data, indent, flags, encoding) && fclose(file.release()) == 0;
 	}
 
 	PUGI__FN bool xml_document::save_file(const wchar_t* path_, const char_t* indent, unsigned int flags, xml_encoding encoding) const
 	{
 		using impl::auto_deleter; // MSVC7 workaround
 		auto_deleter<FILE> file(impl::open_file_wide(path_, (flags & format_save_file_text) ? L"w" : L"wb"), impl::close_file);
 
-		return impl::save_file_impl(*this, file.data, indent, flags, encoding);
+		return impl::save_file_impl(*this, file.data, indent, flags, encoding) && fclose(file.release()) == 0;
 	}
 
 	PUGI__FN xml_node xml_document::document_element() const