fs may not be opened

I just hit a problem where I called svn_fs_list_transactions() without a
prior call to svn_fs_open_berkeley(). It segfaulted within DB because we
tried to create a transaction (in begin_trail) using fs->env which was NULL.

I'm totally fine with allowing segfaults due to caller bogosity (and prefer
it rather than litter my code with arg validation), but figured that I'd
raise it in case somebody disagreed.

Cheers,
-g

-- 
Greg Stein, http://www.lyra.org/