BDB: Convert a silent propogation of corrupted keys into an assertion.

The callers don't bother to check for this condition.  It shouldn't ever happen
and if it does something has gone terribly awry.  Just run our malfunction
handler to avoid making a bigger mess.

* subversion/libsvn_fs_base/key-gen.c
  (svn_fs_base__next_key): Just assert instead of returning an empty string.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570778 13f79535-47bb-0310-9956-ffa450edef68