fix(review): address PR #130 review

- proftest: declare the _mkdir_recursive loop index inside the for.
- xmpp/presence: shorten the resource-snapshot comment to one line.
- command/cmd_funcs: report the /vcard remove index error via cons_show_error.
- common: replace g_assert in g_diff_to_gsize with a log_error + return 0
  (moved out of the header into common.c); take get_random_string length as
  size_t instead of asserting non-negative.
- otr: drop the change-narrating comment on the whitespace memmove.

Refs #112

src/command/cmd_funcs.c

@@ -10066,7 +10066,7 @@ cmd_vcard_remove(ProfWin* window, const char* const command, gchar** args)
         int index;
         auto_gchar gchar* err_msg = NULL;
         if (!strtoi_range(args[1], &index, 0, INT_MAX, &err_msg)) {
-            cons_show("%s", err_msg);
+            cons_show_error("%s", err_msg);
             return TRUE;
         }
         vcard_user_remove_element((unsigned int)index);