patch 9.0.0691: lalloc(0) error in listchars test

Problem:    lalloc(0) error in listchars test.
Solution:   Skip generating text for tab if tab_len is zero.

src/drawline.c

@@ -2802,13 +2802,16 @@ win_line(
 						      && wlv.n_extra > tab_len)
 			    tab_len += wlv.n_extra - tab_len;
 # endif
-			// If wlv.n_extra > 0, it gives the number of chars, to
+			if (tab_len > 0)
-			// use for a tab, else we need to calculate the width
+			{
-			// for a tab.
+			    // If wlv.n_extra > 0, it gives the number of
+			    // chars, to use for a tab, else we need to
+			    // calculate the width for a tab.
 			    int tab2_len = mb_char2len(wp->w_lcs_chars.tab2);
 			    len = tab_len * tab2_len;
 			    if (wp->w_lcs_chars.tab3)
-			    len += mb_char2len(wp->w_lcs_chars.tab3) - tab2_len;
+				len += mb_char2len(wp->w_lcs_chars.tab3)
+								    - tab2_len;
 			    if (wlv.n_extra > 0)
 				len += wlv.n_extra - tab_len;
 			    c = wp->w_lcs_chars.tab1;
@@ -2831,8 +2834,10 @@ win_line(
 					break;
 				    }
 
-				// if tab3 is given, use it for the last char
+				    // if tab3 is given, use it for the last
-				if (wp->w_lcs_chars.tab3 && i == tab_len - 1)
+				    // char
+				    if (wp->w_lcs_chars.tab3
+							   && i == tab_len - 1)
 					lcs = wp->w_lcs_chars.tab3;
 				    p += mb_char2bytes(lcs, p);
 				    wlv.n_extra += mb_char2len(lcs)
@@ -2840,13 +2845,15 @@ win_line(
 				}
 				wlv.p_extra = wlv.p_extra_free;
 # ifdef FEAT_CONCEAL
-			    // n_extra will be increased by FIX_FOX_BOGUSCOLS
+				// n_extra will be increased by
-			    // macro below, so need to adjust for that here
+				// FIX_FOX_BOGUSCOLS macro below, so need to
+				// adjust for that here
 				if (wlv.vcol_off > 0)
 				    wlv.n_extra -= wlv.vcol_off;
 # endif
 			    }
 			}
+		    }
 #endif
 #ifdef FEAT_CONCEAL
 		    {

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    691,
 /**/
     690,
 /**/