Missing CAMLparam in win32's Unix.stat (ocaml#11737)

The `path` argument is used after a `caml_enter_blocking_section`
when the path does not exists (the path is used to build the
unix error exception).

Unfortunately, during the blocking section we may "yield" to
a thread that can trigger a garbage collection and move the
content of `path` elsewhere, triggering a segfault.

The CAMLparam is therefore needed in that case.

Author: mlasson

Changes

@@ -228,6 +228,10 @@ Working version
 - #11732: Ensure that types from packed modules are always generalised
   (Stephen Dolan and Leo White, review by Jacques Garrigue)
 
+- #11737: Fix segfault condition in Unix.stat under Windows in the presence of
+  multiple threads.
+  (Marc Lasson, Nicolás Ojeda Bär, review by Gabriel Scherer and David Allsopp)
+
 OCaml 5.0
 ---------
 

otherlibs/unix/stat_win32.c

@@ -339,50 +339,54 @@ static int do_stat(int do_lstat, int use_64, const char* opath, HANDLE fstat, __
 
 CAMLprim value caml_unix_stat(value path)
 {
+  CAMLparam1(path);
   struct _stat64 buf;
   __int64 st_ino;
 
   caml_unix_check_path(path, "stat");
   if (!do_stat(0, 0, String_val(path), NULL, &st_ino, &buf)) {
     caml_uerror("stat", path);
   }
-  return stat_aux(0, st_ino, &buf);
+  CAMLreturn (stat_aux(0, st_ino, &buf));
 }
 
 CAMLprim value caml_unix_stat_64(value path)
 {
+  CAMLparam1(path);
   struct _stat64 buf;
   __int64 st_ino;
 
   caml_unix_check_path(path, "stat");
   if (!do_stat(0, 1, String_val(path), NULL, &st_ino, &buf)) {
     caml_uerror("stat", path);
   }
-  return stat_aux(1, st_ino, &buf);
+  CAMLreturn (stat_aux(1, st_ino, &buf));
 }
 
 CAMLprim value caml_unix_lstat(value path)
 {
+  CAMLparam1(path);
   struct _stat64 buf;
   __int64 st_ino;
 
   caml_unix_check_path(path, "lstat");
   if (!do_stat(1, 0, String_val(path), NULL, &st_ino, &buf)) {
     caml_uerror("lstat", path);
   }
-  return stat_aux(0, st_ino, &buf);
+  CAMLreturn (stat_aux(0, st_ino, &buf));
 }
 
 CAMLprim value caml_unix_lstat_64(value path)
 {
+  CAMLparam1(path);
   struct _stat64 buf;
   __int64 st_ino;
 
   caml_unix_check_path(path, "lstat");
   if (!do_stat(1, 1, String_val(path), NULL, &st_ino, &buf)) {
     caml_uerror("lstat", path);
   }
-  return stat_aux(1, st_ino, &buf);
+  CAMLreturn (stat_aux(1, st_ino, &buf));
 }
 
 static value do_fstat(value handle, int use_64)