Fix #4108 (Bug when convertinf an LRF file to MOBI)

Changelog.yaml

@@ -55,6 +55,9 @@
     - title: "LIT Input: Remove more invalid markup present in LIT files created by Microsoft Word plugins"
       tickets: [4087]
 
+    - title: "LRF Input: Workaround buggy LRF files with unbalanced closing tags"
+      tickets: [4108]
+
   new recipes:
     - title: The Economist (no subscription required)
       author: Kovid Goyal

src/calibre/ebooks/lrf/objects.py

@@ -872,8 +872,9 @@ class Text(LRFStream):
             if isinstance(c, basestring):
                 s += c
             elif c is None:
-                p = open_containers.pop()
-                s += u'</%s>'%(p.name,)
+                if open_containers:
+                    p = open_containers.pop()
+                    s += u'</%s>'%(p.name,)
             else:
                 s += unicode(c)
                 if not c.self_closing: