files: don't close a newly-created buffer when it is the only one

This fixes https://savannah.gnu.org/bugs/?56504.
Reported-by: Liu Hao <lh_mouse@126.com>

Bug existed since commit 7e422402 from two weeks ago.

src/files.c

@@ -462,7 +462,8 @@ bool open_buffer(const char *filename, bool new_buffer)
 				/* When not overriding an existing lock, discard the buffer. */
 				if (do_lockfile(realname) < 0) {
 #ifdef ENABLE_MULTIBUFFER
-					close_buffer();
+					if (openfile != openfile->next)
+						close_buffer();
 #endif
 					free(realname);
 					return FALSE;