patch 8.1.2189: syntax highlighting wrong for tab

Problem:    Syntax highlighting wrong for tab.
Solution:   Don't clear syntax attribute n_extra is non-zero.
diff --git a/src/drawline.c b/src/drawline.c
index a13c330..ac37928 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -1400,9 +1400,9 @@
 #endif
 
 #ifdef FEAT_SYN_HL
-	    syntax_attr = 0;
 	    if (extra_check && n_extra == 0)
 	    {
+		syntax_attr = 0;
 # ifdef FEAT_TERMINAL
 		if (get_term_attr)
 		    syntax_attr = term_get_attr(wp->w_buffer, lnum, vcol);