If we can't get the vmode, don't do sixel stuff.

--coverity

src/conio/cterm.c

@@ -3105,6 +3105,10 @@ static void parse_sixel_intro(struct cterminal *cterm)
 
 		GETTEXTINFO(&ti);
 		vmode = find_vmode(ti.currmode);
+		if (vmode == -1) {
+			cterm->sixel = SIXEL_INACTIVE;
+			return;
+		}
 		attr2palette(ti.attribute, &cterm->sx_fg, &cterm->sx_bg);
 		if (cterm->sx_scroll_mode) {
 			cterm->sx_x = cterm->sx_left = (cterm->x + WHEREX() - 2) * vparams[vmode].charwidth;