fix: Improve json parsing of currency issuers (#29) (#57)

---------

Co-authored-by: Ayaz Salikhov <asalikhov@ripple.com>
Co-authored-by: Vladislav Vysokikh <vvysokikh@gmail.com>

src/libxrpl/protocol/Issue.cpp

@@ -142,7 +142,7 @@ issueFromJson(Json::Value const& v)
     }
     auto const issuer = parseBase58<AccountID>(issStr.asString());
 
-    if (!issuer)
+    if (!issuer || *issuer == noAccount() || *issuer == xrpAccount())
     {
         Throw<Json::error>("issueFromJson issuer must be a valid account");
     }