Fix xpalloc confusion after memory is exhausted.

Fix xpalloc confusion after memory is exhausted.

* alloc.c (xpalloc): Comment fix.
* charset.c (Fdefine_charset_internal): If xpalloc exhausts memory
and signals an error, do not clear charset_table_size, as
charset_table is still valid.
* doprnt.c (evxprintf): Clear *BUF after freeing it.