write_idx: fix handling of signed/unsigned values

write_idx: fix handling of signed/unsigned values

Coerce the incoming Py_ssize_t count directly to the required uint32_t
while checking for overflow.  The underlying problem was identified
when we hit two sign-compare warnings on (32-bit) platforms where
ssize_t is 32 bits.

Explicitly add -Wsign-comapare to CFLAGS, just to be sure it's
included.

Reported-by: Dominique Leuenberger
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>