man: Fix covscan reports in doxyxml.c

This fixes most of the remaining covscan errors in doxyxml.c.
The ones that remain are caused by malloced structures being
stored in qb_hashtable_maps.

These still cause unfreed memory, because the contents of the maps
are never explictly freed, but as they are used until the very end of
the program (when the OS will free everything) I'm dubious as to
whether it's worth doing it in the code - or whether covscan will
work out what's going on anyway.