Sync'ing FSX with FSFS:
Merge r1559870,1575628,1576801 from /subversion/libsvn_fs_fs into
subversion/libsvn_fs_x.  Conflicts were due structural differences
between FSX and FSFS.

This ports index file bounds checking patches.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595579 13f79535-47bb-0310-9956-ffa450edef68