Follow-up to r1628093: Fix test failures when reading past EOF of a
proto-index file.

* subversion/libsvn_fs_fs/index.c
  (read_uint32_from_proto_index,
   read_off_t_from_proto_index): Verify value ranges and return values only
                                 if a value has actually been read.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628159 13f79535-47bb-0310-9956-ffa450edef68