Rework DSA_sign() and DSA_verify()

Rework DSA_sign() and DSA_verify()

Change DSA_sign() to single exit and check the signed i2d_DSA_SIG() return
value before assigning it to an unsigned int.

In DSA_verify() let d2i_DSA_SIG() handle the allocation, split error check
of i2d_DSA_SIG() from signature check and change an unnecessary freezero()
to free.

ok jsing