Fix for possible division by zero when running headless server

[ceeac]

The code which has the error (in banner_text::draw, line 201/202) is compiled but not executed when using the posix backend, but better be safe than sorry.

[prissi]

Hmm, one could say the other way round, if there is code executing for graphicless server depending on font height. But either is fine with me. Changed in r9079.