Fix clear_trackhostexits_mapping() to actually work as advertised

Previously, it would remove every trackhostexits-derived mapping
*from* xyz.<exitname>.exit; it was supposed to remove every
trackhostexits-derived mapping *to* xyz.<exitname>.exit.

Bugfix on 0.2.0.20-rc: fixes an XXX020 added while staring at bug-1090
issues.

changes/clear_trackexithost

@@ -0,0 +1,5 @@
+  o Minor bugfixes:
+    - Fix a bug in the code where we could keep trying to use a
+      TrackHostExits-based mapping after we failed to reach the intended
+      destination node.  Bugfix on 0.2.0.20-rc.
+

src/or/connection_edge.c

@@ -799,8 +799,8 @@ clear_trackexithost_mappings(const char *exitname)
   tor_strlower(suffix);
 
   STRMAP_FOREACH_MODIFY(addressmap, address, addressmap_entry_t *, ent) {
-    /* XXXX022 HEY!  Shouldn't this look at ent->new_address? */
-    if (ent->source == ADDRMAPSRC_TRACKEXIT && !strcmpend(address, suffix)) {
+    if (ent->source == ADDRMAPSRC_TRACKEXIT &&
+        !strcmpend(ent->new_address, suffix)) {
       addressmap_ent_remove(address, ent);
       MAP_DEL_CURRENT(address);
     }