Return error when importmulti called with invalid address.

Lack of error checking noticed by Alex Morcos <morcos@chaincode.com>

src/wallet/rpcdump.cpp

@@ -671,6 +671,9 @@ UniValue processImport(const UniValue& data) {
 
         if (!isScript) {
             address = CBitcoinAddress(output);
+            if (!address.IsValid()) {
+                throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid address");
+            }
             script = GetScriptForDestination(address.Get());
         } else {
             if (!IsHex(output)) {