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)

src/testdir/test_functions.vim

@@ -2169,6 +2169,8 @@ func Test_charclass()
   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()