Fix warnings for potentially undefined operation.
 
Some code uses increment operators on variables referenced in the same
operation. See fsfs-improvements branch (r1517479,1506545) and
fsfs-format-7 (r1453765)
 
* subversion/libsvn_fs_fs/id.c
  (txn_id_parse): Increment data pointer in a separate statement.
* subversion/libsvn_fs_fs/transaction.c
  (read_next_ids): Increment str pointer in a separate statement.
* subversion/libsvn_fs_x/transaction.c
  (read_next_ids): Same.

Patch by: Andreas Stieger <andreas.stieger{_AT_}gmx.de>


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1593795 13f79535-47bb-0310-9956-ffa450edef68