The efforts to eliminate the dual-representation of zero and to ensure
bignums are passed in and out of functions and APIs in a consistent form has highlighted that zero-valued bignums don't need any allocated word data. The use of BN_set_word() to initialise a bignum to zero causes needless allocation and gives it a return value that must be checked. This change converts BN_zero() to a self-contained macro that has no return/expression value and does not cause any expansion of bignum data. Note, it would be tempting to rewrite the deprecated version as a success-valued comma expression, such as; #define BN_zero(a) ((a)->top = (a)->neg = 0, 1) However, this evaluates 'a' twice and would confuse initialisation loops (eg. while(..) { BN_zero(bn++) } ). As such, the deprecated version continues to use BN_set_word().
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