Bug 359201 followup. futex skips argument 5 if op is FUTEX_WAIT_BITSET.

Bug 359201 followup. futex skips argument 5 if op is FUTEX_WAIT_BITSET.

The original fix in svn r15793 read argument 6 separately by using PRA6
unconditionally. This is wrong. We need to first check whether a
track_pre_reg_read callback is registered (only memcheck does).
The PRE_REG_READX macro already had this check. Just add the same
before calling PRA6. Thanks to Tom Hughes for noticing. Without this
helgrind/tests/cond_timedwait_test and drd/tests/pth_inconsistent_cond_wait
regtests would fail.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15795