text: fix critical in paste_received

Here we calculate the length only in the truncate_multiline condition.
Then we pass pos - 1 to gtk_accessible_text_update_contents() as the end
position, triggering this critical that checks to ensure start <= end.
Fix it by always calculating the length of the string that we insert.

This is the first bug fixed as a result of enabling fatal criticals by
default in Epiphany! 🎉

Fixes #6734

gtk/gtktext.c

@@ -5417,6 +5417,8 @@ paste_received (GObject      *clipboard,
 
   if (priv->truncate_multiline)
     length = truncate_multiline (text);
+  else
+    length = strlen (text);
 
   begin_change (self);
   if (priv->selection_bound != priv->current_pos)