Avoid infinite loop on parsing DSA private keys

Avoid infinite loop on parsing DSA private keys

DSA private keys with ill-chosen g could cause an infinite
loop on deserializing. Add a few sanity checks that ensure
that g is according to the FIPS 186-4: check 1 < g < p and
g^q == 1 (mod p). This is enough to ascertain that g is a
generator of a multiplicative group of order q once we know
that q is prime (which is checked a bit later).

Issue reported with reproducers by Hanno Boeck.
Additional variants and analysis by David Benjamin.

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