[lldb] Host::ShellExpandArguments - fix error check for valid dictionary

Fix repeated check for a valid JSON parse and actually check the dictionary pointer

Reported here: https://pvs-studio.com/en/blog/posts/cpp/1003/ (N40)

lldb/source/Host/windows/Host.cpp

@@ -245,7 +245,7 @@ Status Host::ShellExpandArguments(ProcessLaunchInfo &launch_info) {
     }
 
     auto dict_sp = data_sp->GetAsDictionary();
-    if (!data_sp) {
+    if (!dict_sp) {
       error.SetErrorString("invalid JSON");
       return error;
     }