Add length check for CExtKey deserialization

src/key.h

@@ -172,6 +172,8 @@ struct CExtKey {
     {
         unsigned int len = ::ReadCompactSize(s);
         unsigned char code[BIP32_EXTKEY_SIZE];
+        if (len != BIP32_EXTKEY_SIZE)
+            throw std::runtime_error("Invalid extended key size\n");
         s.read((char *)&code[0], len);
         Decode(code);
     }