patch 8.2.1764: Vim9: no error when assigning to script var with wrong type

Problem:    Vim9: no error when assigning to script var with wrong type.
Solution:   Fix off-by-one error. (closes #7028)

src/testdir/test_vim9_assign.vim

@@ -567,6 +567,16 @@ def Test_assignment_vim9script()
     assert_equal(44, t)
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      var n: number
+      def Func()
+        n = 'string'
+      enddef
+      defcompile
+  END
+  CheckScriptFailure(lines, 'E1012: Type mismatch; expected number but got string')
 enddef
 
 def Mess(): string

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1764,
 /**/
     1763,
 /**/

src/vim9compile.c

@@ -4899,7 +4899,7 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 			{
 			    scriptvar_idx = get_script_item_idx(scriptvar_sid,
 								rawname, TRUE);
-			    if (scriptvar_idx > 0)
+			    if (scriptvar_idx >= 0)
 			    {
 				scriptitem_T *si = SCRIPT_ITEM(scriptvar_sid);
 				svar_T	     *sv =