Graphviz_draw core dumping when using HTML labels

Bug report:

Are you running the latest graph-tool version?
Do you observe the problem with the current git version?
Are you using Macports or Homebrew? If yes, please submit an issue there instead: https://github.com/Homebrew/brew/issues and https://trac.macports.org/newticket
Did you compile graph-tool manually?
If you answered yes above, did you use the exact same compiler to build graph-tool, boost-python and Python?

When using graphviz_draw() with an HTML vertex label, it core dumps. The graphviz_draw() works without issues otherwise. I tracked the issue to the agstrdup_html() function call (https://git.skewed.de/count0/graph-tool/blob/master/src/graph_tool/draw/graphviz_draw.py#L97).

I am currently using Ubuntu 18.04, Python 3.6 (default), graph-tool 2.28 installed from apt from this repository: http://downloads.skewed.de/apt/bionic.

from graph_tool.all import *

g = Graph()

g.add_vertex(2)
g.add_edge(0, 1)

graphviz_draw(g, vprops={'label': '<x>' })

I would expect both vertices to have the same label x. If I remove the enclosing < >, it works. However, when enclosed with < >, to specify an HTML label, it core dumps - regardless if I place actual HTML tags or not.

# Bug report:
- [X] Are you running the latest `graph-tool` version?
- [ ] Do you observe the problem with the current git version?
- [ ] Are you using Macports or Homebrew? If yes, please submit an issue there instead: https://github.com/Homebrew/brew/issues and https://trac.macports.org/newticket
- [ ] Did you compile `graph-tool` manually?
- [ ] If you answered yes above, did you use the exact same compiler to build `graph-tool`, `boost-python` and `Python`?

When using `graphviz_draw()`  with an HTML vertex label, it core dumps. The `graphviz_draw()` works without issues otherwise. I tracked the issue to the `agstrdup_html()` function call (https://git.skewed.de/count0/graph-tool/blob/master/src/graph_tool/draw/graphviz_draw.py#L97).

I am currently using Ubuntu 18.04, Python 3.6 (default), graph-tool 2.28 installed from apt from this repository:  http://downloads.skewed.de/apt/bionic.

```
from graph_tool.all import *

g = Graph()

g.add_vertex(2)
g.add_edge(0, 1)

graphviz_draw(g, vprops={'label': '<x>' })
```

I would expect both vertices to have the same label `x`. If I remove the enclosing < >, it works. However, when enclosed with < >, to specify an HTML label, it core dumps - regardless if I place actual HTML tags or not.