check-templates: Fix traceback with missing closing tag at end of file.

Previously, this would crash with an unclear traceback in this situation.

tools/check-templates

@@ -174,7 +174,7 @@ def get_html_tag(text, i):
         if text[end] == '"':
             quote_count += 1
         end += 1
-    if text[end] != '>':
+    if end == len(text) or text[end] != '>':
         raise Exception('Tag missing >')
     s = text[i:end+1]
     return s