Ensure render-templates only creates/truncates the "destination" if…

… the template renders successfully.

credsmash/templates.py

@@ -113,11 +113,13 @@ def cmd_render_template(
     if not manifest_format:
         manifest_format = detect_format(manifest, 'json')
     for entry in parse_manifest(manifest, manifest_format):
-        with codecs.open(entry['source'], 'r', encoding='utf-8') as template, \
-                codecs.open(entry['destination'], 'w', encoding='utf-8') as destination:
+        with codecs.open(entry['source'], 'r', encoding='utf-8') as template:
             output = env.from_string(template.read()).render(**{
                 obj_name: secrets
             })
+        # Only open the file after rendering the template
+        #  as we truncate the file when opening.
+        with codecs.open(entry['destination'], 'w', encoding='utf-8') as destination:
             destination.write(output)
 
         if 'mode' in entry: