don't taint $DB::sub

don't taint $DB::sub

[perl #76872] showed a case where code like the following, run under -d,
would cause $DB::sub to get set:

    $tainted_expression && func()

The tainted expression sets PL_tainted, so calling func() under -d, which
sets $DB::sub, causes it to get tainted.

Consequently any further sub calls would set PL_tainted while getting the
old value of $DB::sub (and cause the new value to be tainted too), and if
the sub was XS, then its code would be executed with PL_tainted set.
It isn't an issue with perl subs as the first nextstate op resets
PL_tainted.