Replace assert()'s with errors.
Replace assert()'s with errors.

* subversion/libsvn_client/checkout.c:
  Do not include assert.h.
  (svn_client__checkout_internal):
    If the path or url arguments are NULL, then return an error.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion