Fix quotes on Windows.

Boost process wchar_t conversion chokes if it's fed an empty string.
This would happen when the user had no alphavantage key. Separate
the process invocation to not present the empty value to boost process.

libgnucash/app-utils/gnc-quotes.cpp

@@ -211,14 +211,23 @@ GncFQQuoteSource::run_cmd (const StrVec& args, const std::string& json_string) c
         boost::asio::io_service svc;
 
         auto input_buf = bp::buffer (json_string);
-        bp::child process (c_cmd, args,
-                           bp::std_out > out_buf,
-                           bp::std_err > err_buf,
-                           bp::std_in < input_buf,
-                           bp::env["ALPHAVANTAGE_API_KEY"]= (m_api_key.empty() ? "" : m_api_key),
-                           svc);
-        svc.run();
-        process.wait();
+	bp::child process;
+	if (m_api_key.empty())
+	    process = bp::child(c_cmd, args,
+				bp::std_out > out_buf,
+				bp::std_err > err_buf,
+				bp::std_in < input_buf,
+				svc);
+	else
+	    process = bp::child(c_cmd, args,
+				bp::std_out > out_buf,
+				bp::std_err > err_buf,
+				bp::std_in < input_buf,
+				bp::env["ALPHAVANTAGE_API_KEY"] = m_api_key,
+				svc);
+
+	svc.run();
+	process.wait();
 
         {
             auto raw = out_buf.get();