Bug 20569

I have a program trying to use libvgl. When VGLInit is called it calls
VGLTextSetFontFile((byte *)0). This causes VGLTextFont to point to some
allocated memory and VGLTextFont->BitmapArray to point to VGLFont which is a
global array defined in text.c.

Later, when my program calls VGLTextSetFontFile VGLTextSetFontFile tries to
free this array causing "free(): warning: junk pointer, too high to make sense"

Also there are a few style(9) things...specifically the use of (type *)0 rather
than NULL.

Fix: 

I guess a flag needs to be set to indicate if the builtin font is being used
or a user set font. There may be a better way.
How-To-Repeat: 
if (VGLInit(VIDEO_MODE) != 0) {
        err(EX_UNAVAILABLE, "failed to initialise video display");
}

if (VGLTextSetFontFile(FONT) != 0) {
	reset_and_err(EX_UNAVAILABLE, "failed to load font (%s)", FONT);
}