vim-patch:8.2.1274: Vim9: no error for missing white space at script level

Problem:    Vim9: no error for missing white space in assignment at script
            level.
Solution:   Check for white space. (closes vim/vim#6495)

63be3d4ba0

Cherry-pick Test_let_errors() change from patch 8.2.0633.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval/vars.c

@@ -231,11 +231,13 @@ static void ex_let_const(exarg_T *eap, const bool is_const)
           expr++;
         }
       }
-      expr = skipwhite(expr + 2);
+      expr += 2;
     } else {
-      expr = skipwhite(expr + 1);
+      expr += 1;
     }
 
+    expr = skipwhite(expr);
+
     if (eap->skip) {
       emsg_skip++;
     }