files: when there is a slash after the dot, then there is no extension

This fixes https://savannah.gnu.org/bugs/?61071.

Bug existed since commit f429ebe3 from three days ago.

src/files.c

@@ -1467,7 +1467,7 @@ char *safe_tempfile(FILE **stream)
 
 	extension = strrchr(openfile->filename, '.');
 
-	if (!extension)
+	if (!extension || strchr(extension, '/'))
 		extension = openfile->filename + strlen(openfile->filename);
 
 	tempfile_name = nrealloc(tempdir, strlen(tempdir) + 12 + strlen(extension));