help: don't crash in keyword search if an extension fails to provide docs

Not all external extensions provide docs; if you use such an extension, you
will experience a crash if you use "hg help --keyword <word>", and <word>
happens to match the extension name.

mercurial/help.py

@@ -149,6 +149,8 b' def topicmatch(ui, kw):'
     for name, docs in itertools.chain(
         extensions.enabled(False).iteritems(),
         extensions.disabled().iteritems()):
+        if not docs:
+            continue
         mod = extensions.load(ui, name, '')
         name = name.rpartition('.')[-1]
         if lowercontains(name) or lowercontains(docs):