Fix / silence some integer conversion warnings.

* subversion/libsvn_fs_fs/index.c
  (packed_stream_read): We know those are well within apr_size_t's
                        value range.
  (auto_open_l2p_index): The config reader limits the BLOCK_SIZE
                         to apr_size_t since r1628392.
  (get_l2p_header_body): Use the appropriate type for the iteration
                         variable.  Use maximally widening casts
                         for non-negative values where appropriate.
                         Cast revnum count after we verified its range
                         to be within MAX_FILES_PER_DIR.
  (prefetch_l2p_pages): Exit early for non-positive ranges and use
                        maximally widening casts for non-negative
                        values where appropriate.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628393 13f79535-47bb-0310-9956-ffa450edef68