Make share_bmap() do nothing for sharing with oneself

Before, it overwrote '?', '.', ' ' in the bmap with the capitalized
country letter, but only for sectors the player owns.  Pretty
harmless, just weird.  It can't happen currently, because sharebmap
with self fails with "does not have friendly relations towards you".

src/lib/subs/maps.c

@@ -420,6 +420,9 @@ share_bmap(natid from, natid to, struct nstr_sect *ns, char des,
     char todes;
     char from_des = *from_name;
 
+    if (from == to)
+	return 0;
+
     if (isalpha(from_des))
 	from_des &= ~0x20;