patch 8.2.4468: Coverity warns for uninitialized struct member

Problem:    Coverity warns for uninitialized struct member.
Solution:   Set color.index to zero.

src/terminal.c

@@ -4193,6 +4193,7 @@ set_vterm_palette(VTerm *vterm, long_u *rgb)
 	color.red = (unsigned)(rgb[index] >> 16);
 	color.green = (unsigned)(rgb[index] >> 8) & 255;
 	color.blue = (unsigned)rgb[index] & 255;
+	color.index = 0;
 	vterm_state_set_palette_color(state, index, &color);
     }
 }