Replace an assert() with an error.
Replace an assert() with an error.

* subversion/libsvn_client/externals.c:
  Do not include assert.h.
  (handle_external_item_change):
    Do not assert if the old and new item are both NULL, return an
      error.

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