url-mode: fix crash when removing duplicate and/or overlapping URLs

Removing overlaping and duplicated URLs is done by running two nested
loops, that both iterate the same URL list.

When a duplicate is found, one of the URLs is destroyed and removed
from the list.

Deleting and removing an item *is* safe, but only as long as _no
other_ iterator has references to it.

In this case, if we remove an item from e.g. the inner iterator, we’ll
crash if the outer iterator’s *next* item is the item being removed.

Closes #627

terminal.h

@@ -264,6 +264,7 @@ struct url {
     enum url_action action;
     bool url_mode_dont_change_url_attr; /* Entering/exiting URL mode doesn’t touch the cells’ attr.url */
     bool osc8;
+    bool duplicate;
 };
 typedef tll(struct url) url_list_t;