Continue hunting for the cause of the Tru64 failed threaded smokes

Continue hunting for the cause of the Tru64 failed threaded smokes
most probably introduced by the change #19157.  The tweak suggested
by Stephen McCamant trades a possible dangling pointer (CopFILE()
is tricky like that with threads) to a memory leak.  (This means
that this tweak shouldn't be left in as-is, but if this helps,
we at least know that the failure was caused by the #19157.)
p4raw-link: @19157 on //depot/perl: 3871c2ef9aa1dfeba4631a17efdbf486012d47a6

p4raw-id: //depot/perl@19197