Consistently call BN_init() before BN_with_flags()

Consistently call BN_init() before BN_with_flags()

BN_with_flags() preserves the BN_FLG_MALLOCED flag of the destination
which results in a potential use of an uninitialized bit. In practice
this doesn't matter since we don't free the cloned BIGNUMs anyway.

As jsing points out, these are mostly pointless noise and should be
garbage collected. I'll leave that for another rainy day.

Coverity flagged one instance BN_gcd_no_branch(), the rest was found by
the ever so helpful grep(1).

CID 345122

ok jsing