Do not return negative width for text length

[radarhere]

Pillow/src/_imaging.c

Lines 2774 to 2783 in 24696af

	static inline int
	textwidth(ImagingFontObject *self, const unsigned char *text) {
	int xsize;
	for (xsize = 0; *text; text++) {
	xsize += self->glyphs[*text].dx;
	}
	return xsize;
	}

It is possible for dx to be negative. While there's probably scenarios where that makes sense for an individual glyph, when it comes to the total text width, I don't think it makes sense to say that that is negative.

This function determines the value returned by getlength(). I think it's more helpful to return zero. This is likely to be used for a drawing operation, and if nothing would be drawn, then what's the difference?