patch 9.0.0184: virtual text prop highlight continues after truncation

Problem:    Virtual text prop highlight continues after truncation.
Solution:   Recompute the length of attributes.
diff --git a/src/drawline.c b/src/drawline.c
index 0300f37..5d9dcc3 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -1793,6 +1793,7 @@
 					wlv.p_extra = p_extra_free2 = l;
 					wlv.n_extra = n_used + added;
 					n_attr_skip = added;
+					n_attr = mb_charlen(wlv.p_extra);
 				    }
 				}