[PATCH] Fix undefined behavior when constructing ID for txn_node_cache in fsfs

Hello.

I think I have found an undefined behavior in the code that constructs ID for
per-transaction DAG node cache.

In make_txn_root() function, the txn variable of type svn_fs_fs__id_part_t *
is passed to apr_pstrcat() function, leading to the undefined behavior.
I can assume that originally it was planned to pass a textual representation
of the txn, instead of passing the txn itself.

Although ID is not used anywhere, except debug-only calls of
svn_cache__get_info() and svn_cache__format_info() functions, the improper call
to apr_pstrcat() may cause a potential crash, etc.

I have attached a patch that fixes it.

Regards,
Denis Kovalchuk