[dev] [dmenu] Handle invalid font names

[Ilias Tsitsimpis]

Hi suckless team,

I noticed that dmenu doesn't complain when supplied with a font that
doesn't exist (e.g., dmenu -fn suckless).

I found the following line in the code:

    drw.c:134  fprintf(stderr, "error, cannot load font: '%s'\n", fontname);

so I am guessing this is a bug and not the intended behavior.

Keep up the good work!

Cheers,
Ilias