fix bug introduced in 21673c95f4

lib/wallet.py

@@ -139,6 +139,10 @@ class WalletStorage(object):
         s = json.dumps(self.data, indent=4, sort_keys=True)
         with open(temp_path, "w") as f:
             f.write(s)
+        # perform atomic write on POSIX systems
+        try:
+            os.rename(temp_path, self.path)
+        except:
             os.remove(self.path)
             os.rename(temp_path, self.path)
         if 'ANDROID_DATA' not in os.environ: