Rework DSA_size() and ECDSA_size()

Rework DSA_size() and ECDSA_size()

DSA_size() and ECDSA_size() have a very special hack. They fudge up an
ASN1_INTEGER with a size which is typically > 100 bytes, backed by a
buffer of size 4. This was "fine", however, since they set buf[0] = 0xff,
where the craziness that was i2c_ASN1_INTEGER() only looks at the first
octet (one may then ask why a buffer of size 4 was necessary...).

This changed with the rewrite of i2c_ASN1_INTEGER(), which doesn't
respect this particular hack and rightly assumes that it is fed an
actual ASN1_INTEGER...

Instead, create an appropriate signature and use i2d to determine its
size.

Fixes an out-of-bounds read flagged by ASAN and oss-fuzz.

ok jsing