patch 8.2.3083: crash when passing null string to charclass()

Problem:    Crash when passing null string to charclass().
Solution:   Bail out when string pointer is NULL. (Christian Brabandt,
            closes #8498, closes #8260)
diff --git a/src/mbyte.c b/src/mbyte.c
index 8ac15d3..b2519ec 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -5587,7 +5587,8 @@
     void
 f_charclass(typval_T *argvars, typval_T *rettv UNUSED)
 {
-    if (check_for_string_arg(argvars, 0) == FAIL)
+    if (check_for_string_arg(argvars, 0) == FAIL
+	    || argvars[0].vval.v_string == NULL)
 	return;
     rettv->vval.v_number = mb_get_class(argvars[0].vval.v_string);
 }
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 23af532..3164ca2 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -2169,6 +2169,8 @@
   call assert_equal(1, charclass('.'))
   call assert_equal(2, charclass('x'))
   call assert_equal(3, charclass("\u203c"))
+  " this used to crash vim
+  call assert_equal(0, "xxx"[-1]->charclass())
 endfunc
 
 func Test_eventhandler()
diff --git a/src/version.c b/src/version.c
index 532e747..540fa2b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3083,
+/**/
     3082,
 /**/
     3081,