Improve robustness of DER recoding code

Add some defensive programming on top of #5634.

This copies the respective OpenSSL code in ECDSA_verify in
OpenSSL pre-1.0.1k (e.g. https://github.com/openssl/openssl/blob/OpenSSL_1_0_1j/crypto/ecdsa/ecs_vrf.c#L89)
more closely.

As reported by @sergiodemianlerner.

Bitcoin Github-Pull: #5640
Rebased-From: c6b7b29f23

src/key.cpp

@@ -209,7 +209,18 @@ public:
         unsigned char *norm_der = NULL;
         ECDSA_SIG *norm_sig = ECDSA_SIG_new();
         const unsigned char* sigptr = &vchSig[0];
-        d2i_ECDSA_SIG(&norm_sig, &sigptr, vchSig.size());
+        assert(norm_sig);
+        if (d2i_ECDSA_SIG(&norm_sig, &sigptr, vchSig.size()) == NULL)
+        {
+            /* As of OpenSSL 1.0.0p d2i_ECDSA_SIG frees and nulls the pointer on
+             * error. But OpenSSL's own use of this function redundantly frees the
+             * result. As ECDSA_SIG_free(NULL) is a no-op, and in the absence of a
+             * clear contract for the function behaving the same way is more
+             * conservative.
+             */
+            ECDSA_SIG_free(norm_sig);
+            return false;
+        }
         int derlen = i2d_ECDSA_SIG(norm_sig, &norm_der);
         ECDSA_SIG_free(norm_sig);
         if (derlen <= 0)