Fixed crash when loading non-font bytes

[radarhere]

Resolves #3853, alternative to #3854

The issue reports that loading a non-font as bytes into ImageFont.truetype causes a crash. The alternative PR concludes that it is because of memory being free twice - one in getfont, and once in font_dealloc.

Pillow/src/_imagingft.c

Lines 316 to 318 in 9d5a50a

	if (self->font_bytes) {
	PyMem_Free(self->font_bytes);
	}

Pillow/src/_imagingft.c

Lines 1034 to 1041 in 9d5a50a

	font_dealloc(FontObject* self)
	{
	if (self->face) {
	FT_Done_Face(self->face);
	}
	if (self->font_bytes) {
	PyMem_Free(self->font_bytes);
	}

The alternative PR solves this by removing the PyMem_Free from getfont. This PR suggests instead setting self->font_bytes to null after the memory is freed the first time.

Also, the alternative PR points out that once the error is raised, os.path.basename(font) is called - which doesn't work if font is a file-like object. So this PR also fixes that.

[hugovk]

Thanks, let's go with this one as it's a smaller change.