drw_text: don't segfault when called with 0 width

this patch just rejects *any* 0 width draws, which is surely an error by
the caller.

this also guards against cases where the width is too small for the
ellipsis to fit, so ellipsis_w will remain 0.
reported by Bakkeby <bakkeby@gmail.com>

drw.c

@@ -267,7 +267,7 @@ drw_text(Drw *drw, int x, int y, unsigned int w, unsigned int h, unsigned int lp
 	enum { nomatches_len = 64 };
 	static struct { long codepoint[nomatches_len]; unsigned int idx; } nomatches;
 
-	if (!drw || (render && !drw->scheme) || !text || !drw->fonts)
+	if (!drw || (render && (!drw->scheme || !w)) || !text || !drw->fonts)
 		return 0;
 
 	if (!render) {