am d82ab38c: Ensure BN_asc2bn, BN_dec2bn, and BN_hex2bn never give -0.

* commit 'd82ab38ca2b63638a2cb0b5d8a2c76d90c86dd31':
  Ensure BN_asc2bn, BN_dec2bn, and BN_hex2bn never give -0.