Fix false Valgrind warning.

On some compiler/platform combination, an assignment of form *p= *p
would map to a memcpy() call, and Valgrind flags this as an overlapped
memcpy() error.

Fix by prefixing *p= *q with if(p!=q) when building for Valgrind
(HAVE_purify).

sql/sql_select.cc

@@ -3520,6 +3520,10 @@ update_ref_and_keys(THD *thd, DYNAMIC_ARRAY *keyuse,JOIN_TAB *join_tab,
 	  continue;
       }
 
+#ifdef HAVE_purify
+      /* Valgrind complains about overlapped memcpy when save_pos==use. */
+      if (save_pos != use)
+#endif
         *save_pos= *use;
       prev=use;
       found_eq_constant= !use->used_tables;