Fix ASAN failure in main.lock (and others)

Whenever one copies an IO_CACHE struct, one must remember to call
setup_io_cache, if not, the IO_CACHE's current_pos and end_pos
self-references will point to the previous struct's memory, which
could go out of scope. Commit 90038693
fixes this problem in a more general fashion by removing the
self-references altogether, but for 5.5 we'll keep the old behaviour.

sql/sql_update.cc

@@ -628,6 +628,8 @@ int mysql_update(THD *thd,
       if (reinit_io_cache(&tempfile,READ_CACHE,0L,0,0))
 	error=1; /* purecov: inspected */
       select->file=tempfile;			// Read row ptrs from this file
+      // select->file was copied, update self-references.
+      setup_io_cache(&select->file);
       if (error >= 0)
 	goto err;
     }