simplify check in hdnode_from_xpub

bip32.c

@@ -33,8 +33,7 @@
 
 int hdnode_from_xpub(uint32_t depth, uint32_t fingerprint, uint32_t child_num, const uint8_t *chain_code, const uint8_t *public_key, HDNode *out)
 {
-	curve_point c;
-	if (!ecdsa_read_pubkey(public_key, &c)) { // invalid pubkey
+	if (public_key[0] != 0x02 && public_key[0] != 0x03) { // invalid pubkey
 		return 0;
 	}
 	out->depth = depth;