patch 8.2.4049: Vim9: reading before the start of the line with "$"

Problem:    Vim9: reading before the start of the line with "$" by itself.
Solution:   Do not subtract one when reporting the error.

src/testdir/test_vim9_expr.vim

@@ -2853,6 +2853,7 @@ def Test_expr7_environment()
   CheckDefAndScriptSuccess(lines)
 
   CheckDefAndScriptFailure(["var x = $$$"], ['E1002:', 'E15:'], 1)
+  CheckDefAndScriptFailure(["$"], ['E1002:', 'E15:'], 1)
 enddef
 
 def Test_expr7_register()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4049,
 /**/
     4048,
 /**/

src/vim9expr.c

@@ -1233,7 +1233,7 @@ compile_get_env(char_u **arg, cctx_T *cctx)
     len = get_env_len(arg);
     if (len == 0)
     {
-	semsg(_(e_syntax_error_at_str), start - 1);
+	semsg(_(e_syntax_error_at_str), start);
 	return FAIL;
     }