patch 8.1.2168: heredoc assignment not skipped in if block

Problem:    Heredoc assignment not skipped in if block.
Solution:   Check if "skip" is set. (closes #5063)

src/evalvars.c

@@ -756,10 +756,13 @@ ex_let_const(exarg_T *eap, int is_const)
 	if (l != NULL)
 	{
 	    rettv_list_set(&rettv, l);
-	    op[0] = '=';
+	    if (!eap->skip)
-	    op[1] = NUL;
+	    {
-	    (void)ex_let_vars(eap->arg, &rettv, FALSE, semicolon, var_count,
+		op[0] = '=';
+		op[1] = NUL;
+		(void)ex_let_vars(eap->arg, &rettv, FALSE, semicolon, var_count,
 								is_const, op);
+	    }
 	    clear_tv(&rettv);
 	}
     }

src/testdir/test_let.vim

@@ -295,4 +295,12 @@ E
   app
   END
   call assert_equal(['something', 'app'], var1)
+
+  let check = []
+  if 0
+     let check =<< trim END
+       from heredoc
+     END
+  endif
+  call assert_equal([], check)
 endfunc

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2168,
 /**/
     2167,
 /**/