Config fix (#6261)

homeassistant/components/config/__init__.py

@@ -129,5 +129,8 @@ def _read(path):
 
 def _write(path, data):
     """Write YAML helper."""
+    # Do it before opening file. If dump causes error it will now not
+    # truncate the file.
+    data = dump(data)
     with open(path, 'w', encoding='utf-8') as outfile:
-        outfile.write(dump(data))
+        outfile.write(data)