vim-patch:8.2.4453: :helpgrep may free an option that was not allocated

Problem:    :helpgrep may free an option that was not allocated. (Yegappan
            Lakshmanan)
Solution:   Check if the value was allocated.
4791fcd825

src/nvim/option.c

@@ -3112,6 +3112,12 @@ void set_option_value_give_err(const char *name, long number, const char *string
   }
 }
 
+bool is_option_allocated(const char *name)
+{
+  int idx = findoption(name);
+  return idx >= 0 && (options[idx].flags & P_ALLOCED);
+}
+
 /// Return true if "name" is a string option.
 /// Returns false if option "name" does not exist.
 bool is_string_option(const char *name)

src/nvim/quickfix.c

@@ -7065,6 +7065,7 @@ void ex_helpgrep(exarg_T *eap)
   bool updated = false;
   // Make 'cpoptions' empty, the 'l' flag should not be used here.
   char *const save_cpo = p_cpo;
+  const bool save_cpo_allocated = is_option_allocated("cpo");
   p_cpo = empty_option;
 
   bool new_qi = false;
@@ -7104,8 +7105,10 @@ void ex_helpgrep(exarg_T *eap)
     if (*p_cpo == NUL) {
       set_option_value_give_err("cpo", 0L, save_cpo, 0);
     }
+    if (save_cpo_allocated) {
       free_string_option(save_cpo);
     }
+  }
 
   if (updated) {
     // This may open a window and source scripts, do this after 'cpo' was

src/nvim/testdir/test_quickfix.vim

@@ -714,6 +714,33 @@ func Test_helpgrep()
   call s:test_xhelpgrep('l')
 endfunc
 
+func Test_helpgrep_restore_cpo_aucmd()
+  let save_cpo = &cpo
+  augroup QF_Test
+    au!
+    autocmd BufNew * set cpo=acd
+  augroup END
+
+  helpgrep quickfix
+  call assert_equal('acd', &cpo)
+  %bw!
+
+  set cpo&vim
+  augroup QF_Test
+    au!
+    autocmd BufReadPost * set cpo=
+  augroup END
+
+  helpgrep buffer
+  call assert_equal('', &cpo)
+
+  augroup QF_Test
+    au!
+  augroup END
+  %bw!
+  let &cpo = save_cpo
+endfunc
+
 func Test_errortitle()
   augroup QfBufWinEnter
     au!