Perl interface: SVN::Core::dirent_canonicalize segfaults on undef

I have reported the following Debian bug:

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=812318

With Subversion 1.9.3:

$ perl -MSVN::Core -e 'SVN::Core::dirent_canonicalize(undef)'
zsh: segmentation fault (core dumped) perl -MSVN::Core -e 'SVN::Core::dirent_canonicalize(undef)'

In case of undef, I suppose that the Perl module should pass the empty
string to the library (for which there are no errors), or return some
other kind of error.

I have not tried other kinds of arguments, but in any case, the
module should make sure that a valid string is passed to the
library.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)