Make the default stream full-read implementation actually work as
advertised. Before this change, it would return the remaining space
in the buffer instead of the number of bytes read.

* subversion/libsvn_subr/stream.c (full_read_fallback): Rewritten from scratch.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569296 13f79535-47bb-0310-9956-ffa450edef68