check for utf8_decode errors (#327)

utf8_decode() may return an errors, in which case the returned codepoint
might be garbage. the issue can be tested by adding the following to
`image-info` which produces invalid utf8 sequences:

	base64 -d << EOF
	9JCAgPSQgIH0kICC9JCAg/SQgIT0kICF9JCAhvSQgIf0kICI9JCAifSQgIr0kICL9JCAjPSQgI30
	kICO9JCAj/SQgJD0kICR9JCAkvSQgJP0kICU9JCAlfSQgJb0kICX9JCAmPSQgJn0kICa9JCAm/SQ
	gJz0kICd9JCAnvSQgJ8=
	EOF

on my system, this leads to the statusbar being filled with empty boxes.

check for returned error and skip the iteration.

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/327
Reviewed-by: Berke Kocaoğlu <berke.kocaoglu@metu.edu.tr>

window.c

@@ -407,7 +407,7 @@ void win_clear(win_t *win)
 static int win_draw_text(win_t *win, XftDraw *d, const XftColor *color,
                          int x, int y, char *text, int len, int w)
 {
-	int err, tw = 0;
+	int err, tw = 0, warned = 0;
 	char *t, *next;
 	uint32_t rune;
 	XftFont *f;
@@ -415,7 +415,14 @@ static int win_draw_text(win_t *win, XftDraw *d, const XftColor *color,
 	XGlyphInfo ext;
 
 	for (t = text; t - text < len; t = next) {
+		err = 0;
 		next = utf8_decode(t, &rune, &err);
+		if (err) {
+			if (!warned)
+				error(0, 0, "error decoding utf8 status-bar text");
+			warned = 1;
+			continue;
+		}
 		if (XftCharExists(win->env.dpy, font, rune)) {
 			f = font;
 		} else { /* fallback font */