Prevent negative zero from being created via BN bit functions.

Prevent negative zero from being created via BN bit functions.

Both BN_clear_bit() and BN_mask_bits() can create zero values - in both
cases ensure that the negative sign is correctly handled if the value
becomes zero.

Thanks to Guido Vranken for providing a reproducer.

Fixes oss-fuzz #67901

ok tb@