Dav: destination length validation for COPY and MOVE.

Dav: destination length validation for COPY and MOVE.

Previously, when alias was used in a location with Dav COPY or MOVE
enabled, and the destination URI was shorter than the alias, integer
underflow could happen in ngx_http_map_uri_to_path(), which could
result in heap buffer overwrite, followed by a possible segfault.
With some implementations of memcpy(), the segfault could be avoided
and the overwrite could result in a change of the source or destination
file names to be outside of the location root.

Reported by Calif.io in collaboration with Claude and Anthropic Research.