patch 8.2.2892: error message contains random characters

Problem:    Error message contains random characters.
Solution:   Pass the right pointer to error_white_both(). (closes #8272,
            closes #8263)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 3ca1f34..f1762d7 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1220,7 +1220,14 @@
   lines =<< trim END
       echo 'a'.. 'b'
   END
-  CheckDefAndScriptFailure(lines, 'E1004:', 1)
+  CheckDefAndScriptFailure(lines, 'E1004: White space required before and after ''..'' at ".. ''b''"', 1)
+
+  lines =<< trim END
+      echo 'a'
+          ..'b'
+      # comment
+  END
+  CheckDefAndScriptFailure(lines, 'E1004: White space required before and after ''..'' at "..''b''"', 2)
 
   # check invalid string concatenation
   lines =<< trim END