valgrind doctree leak

(from Jeremy's logs)

==2186== 928 (32 direct, 896 indirect) bytes in 1 blocks are definitely lost in loss record 815 of 958
==2186==    at 0x40275CC: operator new(unsigned int) (vg_replace_malloc.c:255)
==2186==    by 0x806E258: Doctree::push() (doctree.hh:67)
==2186==    by 0x806C1FC: StyleEngine::startElement(int) (styleengine.cc:93)
==2186==    by 0x8076DB8: Html_process_tag(DilloHtml*, char*, int) (html.cc:3507)
==2186==    by 0x8077A33: Html_write_raw(DilloHtml*, char*, int, int) (html.cc:3822)
==2186==    by 0x8070543: DilloHtml::write(char*, int, int) (html.cc:552)
==2186==    by 0x80775BD: Html_callback(int, _CacheClient*) (html.cc:3725)
==2186==    by 0x80627AD: Cache_process_queue (cache.c:1195)
==2186==    by 0x80629A2: Cache_delayed_process_queue_callback (cache.c:1260)
==2186==    by 0x40C0876: Fl::wait(double) (in /usr/lib/libfltk.so.1.3)
==2186==    by 0x40C0A30: Fl::run() (in /usr/lib/libfltk.so.1.3)
==2186==    by 0x804F08A: main (dillo.cc:436)