patch 8.2.1391: Vim9: no error for shadowing a script function

Problem:    Vim9: no error for shadowing a script function.
Solution:   Check for already defined items. (closes #6652)

src/testdir/test_vim9_script.vim

@@ -1780,6 +1780,18 @@ def Test_func_redefine_fails()
     enddef
   END
   CheckScriptFailure(lines, 'E1073:')
+
+  lines =<< trim END
+    vim9script
+    def Foo(): string
+      return 'foo'
+      enddef
+    def Func()
+      let  Foo = {-> 'lambda'}
+    enddef
+    defcompile
+  END
+  CheckScriptFailure(lines, 'E1073:')
 enddef
 
 def Test_fixed_size_list()

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1391,
 /**/
     1390,
 /**/

src/vim9compile.c

@@ -5462,6 +5462,8 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 			semsg(_(e_unknown_var), name);
 			goto theend;
 		    }
+		    else if (check_defined(var_start, varlen, cctx) == FAIL)
+			goto theend;
 		}
 	    }