Make DSA_sign() test for negative p,q,g values.

Make DSA_sign() test for negative p,q,g values.

Related to #20268

DSA_sign() assumes that the signature passed in is related to DSA_size().
If q is negative then DSA_size() actually fails and returns 0.

A test that tries to allocate the signature buffer using DSA_size() and then
pass it to DSA_sign() will then either.

(1) Have a signature buffer of NULL. In this case it was leaking data
returned via i2d_DSA_SIG.

(2) Cause a seg fault because we created a buffer that was not large
enough to hold the signature. As it already checked zero we also now
check for negative values also.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/20553)