If the input 'bn' is NULL then BN_bin2bn() will allocate and
return a BIGNUM which will then be leaked. BN_set_word()
would then return true even though it didn't set the word
into a BIGNUM known to the caller.
Change-Id: I6681f548e6bfb330cec009ae7c18e6151016179e
19eae8b4a9