Missing CAMLparam in win32's Unix.stat (#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.

(cherry picked from commit 3d8fb96)

Author: mlasson

Changes

@@ -64,6 +64,10 @@ OCaml 4.14 maintenance branch
   mismatch error involving recursive types.
   (Florian Angeletti, review by Gabriel Scherer)
 
+- #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 4.14.0 (28 March 2022)
 ----------------------------
 

otherlibs/win32unix/stat.c

@@ -347,50 +347,54 @@ static int do_stat(int do_lstat, int use_64, const char* opath, HANDLE fstat, __
 
 CAMLprim value 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)) {
     uerror("stat", path);
   }
-  return stat_aux(0, st_ino, &buf);
+  CAMLreturn (stat_aux(0, st_ino, &buf));
 }
 
 CAMLprim value 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)) {
     uerror("stat", path);
   }
-  return stat_aux(1, st_ino, &buf);
+  CAMLreturn (stat_aux(1, st_ino, &buf));
 }
 
 CAMLprim value 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)) {
     uerror("lstat", path);
   }
-  return stat_aux(0, st_ino, &buf);
+  CAMLreturn (stat_aux(0, st_ino, &buf));
 }
 
 CAMLprim value 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)) {
     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)