Do not crash if the sources of a move or copy API call is empty.

* subversion/libsvn_client/copy.c (try_copy):
   Add assertion that the source array contains at least one element.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1526103 13f79535-47bb-0310-9956-ffa450edef68