patch 8.2.0890: no color in terminal window when 'termguicolor' is set

Problem:    No color in terminal window when 'termguicolor' is set.
Solution:   Clear the underline color. (closes #6186)
diff --git a/src/highlight.c b/src/highlight.c
index e446966..852fffb 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -2230,10 +2230,12 @@
 #ifdef FEAT_TERMGUICOLORS
     at_en.ae_u.cterm.fg_rgb = INVALCOLOR;
     at_en.ae_u.cterm.bg_rgb = INVALCOLOR;
+    at_en.ae_u.cterm.ul_rgb = INVALCOLOR;
 #endif
     at_en.ae_attr = attr;
     at_en.ae_u.cterm.fg_color = fg;
     at_en.ae_u.cterm.bg_color = bg;
+    at_en.ae_u.cterm.ul_color = INVALCOLOR;
     return get_attr_entry(&cterm_attr_table, &at_en);
 }
 #endif
@@ -2262,6 +2264,7 @@
 	at_en.ae_u.cterm.fg_rgb = fg;
 	at_en.ae_u.cterm.bg_rgb = bg;
     }
+    at_en.ae_u.cterm.ul_rgb = INVALCOLOR;
     return get_attr_entry(&cterm_attr_table, &at_en);
 }
 #endif