Fix 2 subscript[0] bugs in pubkey.cpp, and eliminate one extra size check

src/pubkey.cpp

@@ -172,10 +172,7 @@ bool CPubKey::Verify(const uint256 &hash, const std::vector<unsigned char>& vchS
     if (!secp256k1_ec_pubkey_parse(secp256k1_context_verify, &pubkey, &(*this)[0], size())) {
         return false;
     }
-    if (vchSig.size() == 0) {
-        return false;
-    }
-    if (!ecdsa_signature_parse_der_lax(secp256k1_context_verify, &sig, &vchSig[0], vchSig.size())) {
+    if (!ecdsa_signature_parse_der_lax(secp256k1_context_verify, &sig, vchSig.data(), vchSig.size())) {
         return false;
     }
     /* libsecp256k1's ECDSA verification requires lower-S signatures, which have
@@ -274,7 +271,7 @@ bool CExtPubKey::Derive(CExtPubKey &out, unsigned int _nChild) const {
 
 /* static */ bool CPubKey::CheckLowS(const std::vector<unsigned char>& vchSig) {
     secp256k1_ecdsa_signature sig;
-    if (!ecdsa_signature_parse_der_lax(secp256k1_context_verify, &sig, &vchSig[0], vchSig.size())) {
+    if (!ecdsa_signature_parse_der_lax(secp256k1_context_verify, &sig, vchSig.data(), vchSig.size())) {
         return false;
     }
     return (!secp256k1_ecdsa_signature_normalize(secp256k1_context_verify, NULL, &sig));