manual: gracefully handle dangling references

Recent doxygen versions generate references for constants that are named
in the text. Since we do not produce a constant table in the manual
(neither markdown nor tex), these references point to nowhere and
produce a KeyError in the ref_dict on lookup.

The corresponding xml nodes are of this form:

<ref kindref="member"
     refid="include_2sel4_2constants_8h_1a9a3...">seL4_TCBFlag</ref>

Ignore such dangling references and return the content (seL4_TCBFlag in
the example) instead. Run text escape on the content to cover
underscores etc in LaTeX.

Does not produce a warning since we expect this to be normal behaviour.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

manual/tools/parse_doxygen_xml.py

@@ -96,10 +96,11 @@ class Generator(object):
         return ""
 
     def ref_to_format(self, para, ref_dict):
-        """Convert a reference by id to a latex command by looking up refid in para"""
-        if len(ref_dict) > 0:
+        """Format a reference by id by looking up refid in para. Return escaped content if not found."""
+        try:
             return self.ref_format(para["refid"], ref_dict)
-        return ""
+        except KeyError:
+            return self.text_escape(para.contents[0])
 
     def nref_to_format(self, para, ref_dict):
         """Convert a reference by name to a latex command by looking up refid in para"""