files: it is fine to "change" the name of a nameless buffer

This fixes https://savannah.gnu.org/bugs/?48156.

src/files.c

@@ -2216,7 +2216,7 @@ int do_writeout(bool exiting)
     append_type append = OVERWRITE;
     char *given;
 	/* The filename we offer, or what the user typed so far. */
-    bool maychange = FALSE;
+    bool maychange = (openfile->filename[0] == '\0');
 	/* Whether it's okay to save the file under a different name. */
 #ifndef DISABLE_EXTRA
     static bool did_credits = FALSE;