Protect against nullptr dereference, remove unused GError.

Found by clang static analyzer.

libgnucash/backend/xml/io-gncxml-v2.cpp

@@ -1539,7 +1539,6 @@ try_gz_open (const char* filename, const char* perms, gboolean use_gzip,
     {
         int filedes[2];
         GThread* thread;
-        GError* error = NULL;
         gz_thread_params_t* params;
         FILE* file;
 
@@ -1564,9 +1563,7 @@ try_gz_open (const char* filename, const char* perms, gboolean use_gzip,
                                params);
         if (!thread)
         {
-            g_warning ("Could not create thread for (de)compression: %s",
-                       error->message);
-            g_error_free (error);
+            g_warning ("Could not create thread for (de)compression.");
             g_free (params->filename);
             g_free (params->perms);
             g_free (params);

libgnucash/backend/xml/io-utils.cpp

@@ -65,6 +65,8 @@ write_one_account (FILE* out,
     xmlElemDump (out, NULL, accnode);
     xmlFreeNode (accnode);
 
+    g_return_val_if_fail(gd, FALSE);
+    
     if (ferror (out) || fprintf (out, "\n") < 0)
         return FALSE;