svn_client_delete4 can segfault

Hi,

from a few crash reports sent in for TSVN I was finally able to pin down
the problem:

in libsvn_client\delete.c, line 240 there's a call
repos_relpath = svn_uri__is_child(repos_root, uri, pool);

but that can return NULL.
further below then that repos_relpath is passed to svn_ra_check_path
which then segfaults.

looking at the code it's possible for repos_relpath to stay NULL, for
example, if the repo root is passed to svn_client_delete4 - which is of
course something that can't be done, but segfaulting is bad, instead an
error should be returned.
And I have to assume that if the url was entered with the wrong case,
svn_uri__is_child() would also return NULL.

on trunk, svn_uri__is_child() has been replaced with
svn_uri_skip_ancestor() (r1198802), but it works the same as
svn_uri__is_child() from what I can see.

Stefan

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