Avoid wraparound when casting unsigned size_t to signed ssize_t.

Avoid wraparound when casting unsigned size_t to signed ssize_t.

Practically, this only affects a perl compiled with 64-bit IVs on a 32-bit
system.  In that instance a value of count >= 2**31 would turn negative
when cast to (ssize_t).
(cherry picked from commit 94e529cc4d56863d7272c254a29eda2b002a4335)