Issue #3657: Fix uninitialized memory read when pickling longs.

The conversion to the unicode API was incorrect, it should use bytes.
repr is a bad variable name.  The use is overloaded, but I'll leave
that to fix later.

R=Brett
TESTED=./python -E -tt ./Lib/test/regrtest.py -uall
	valgrind -q --leak-check=yes --suppressions=Misc/valgrind-python.supp \
		./python -E -tt ./Lib/test/regrtest.py test_pickletools