Avoid printing the typecast name in debug statement

The way we were doing it is unsafe and causes assertion failures on Py3.

Fixes ticket #188

NEWS

@@ -10,6 +10,8 @@ What's new in psycopg 2.5.3
   Chris Withers (:ticket:`#193`).
 - Avoid blocking async connections on connect (:ticket:`#194`). Thanks to
   Adam Petrovich for the bug report and diagnosis.
+- Fixed unsafe access to object names causing assertion failures in
+  Python 3 debug builds (:ticket:`#188`).
 - Fixed handling of dsn and closed attributes in connection subclasses
   failing to connect (from :ticket:`#192` discussion).
 - Fixed overflow opening a lobject with an oid not fitting in a signed int

psycopg/pqpath.c

@@ -1134,9 +1134,8 @@ _pq_fetch_tuples(cursorObject *curs)
             cast = psyco_default_cast;
         }
 
-        Dprintf("_pq_fetch_tuples: using cast at %p (%s) for type %d",
-                cast, Bytes_AS_STRING(((typecastObject*)cast)->name),
-                PQftype(curs->pgres,i));
+        Dprintf("_pq_fetch_tuples: using cast at %p for type %d",
+                cast, PQftype(curs->pgres,i));
         Py_INCREF(cast);
         PyTuple_SET_ITEM(casts, i, cast);