patch 8.2.4468: Coverity warns for uninitialized struct member

Problem:    Coverity warns for uninitialized struct member.
Solution:   Set color.index to zero.
diff --git a/src/terminal.c b/src/terminal.c
index 43a216f..75f7a08 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -4193,6 +4193,7 @@
 	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);
     }
 }