Remove more OOM error handling code

From the functions:

 - ExpandBufnames
 - buf_modname()
 - do_autocmd_event()
 - ff_create_stack_element()
 - ff_get_visited_list()
 - ins_complete()
 - msg_show_console_dialog()
 - prt_find_resource()
 - vim_findfile_init()

TODO: refactor msg_show_console_dialog() to make sure it doesn't ever return
NULL.

src/diff.c

@@ -1403,7 +1403,6 @@ static void diff_read(int idx_orig, int idx_new, char_u *fname)
     notset = TRUE;
   }
 
-done:
   fclose(fd);
 }