patch 8.0.1172: when E734 is given option is still set

Problem:    When E734 is given option is still set.
Solution:   Assign NULL to "s". (Christian Brabandt)

src/eval.c

@@ -1704,7 +1704,10 @@ ex_let_one(
 						       &stringval, opt_flags);
 		if ((opt_type == 1 && *op == '.')
 			|| (opt_type == 0 && *op != '.'))
+		{
 		    EMSG2(_(e_letwrong), op);
+		    s = NULL;  /* don't set the value */
+		}
 		else
 		{
 		    if (opt_type == 1)  /* number */

src/testdir/test_assign.vim

@@ -29,3 +29,17 @@ func Test_let_termcap()
   let &t_xx = ""
   call assert_fails('let x = &t_xx', 'E15')
 endfunc
+
+func Test_let_option_error()
+  let _w = &tw
+  let &tw = 80
+  call assert_fails('let &tw .= 1', 'E734')
+  call assert_equal(80, &tw)
+  let &tw = _w
+
+  let _w = &fillchars
+  let &fillchars = "vert:|"
+  call assert_fails('let &fillchars += "diff:-"', 'E734')
+  call assert_equal("vert:|", &fillchars)
+  let &fillchars = _w
+endfunc

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1172,
 /**/
     1171,
 /**/