Use a dummy public key that has a chance of being an actual public key.

This works around an expected test failure until we add a check that a 
signature can be verified with the provided DSA parameters.

It would probably make more sense to mark the failing test without this
diff as XFAIL.

Index: DSA.xs
--- DSA.xs.orig
+++ DSA.xs
@@ -601,12 +601,12 @@ set_priv_key(dsa, priv_key_SV)
     PREINIT:
         STRLEN len;
         const BIGNUM *old_pub_key;
-        BIGNUM *pub_key;
+        BIGNUM *pub_key = NULL;
         BIGNUM *priv_key;
     CODE:
         DSA_get0_key(dsa, &old_pub_key, NULL);
         if (NULL == old_pub_key) {
-            pub_key = BN_new();
+            BN_dec2bn(&pub_key, "2");
             if (NULL == pub_key) {
                 croak("Could not create a dummy public key");
             }