os: Don't require an on-stack buffer

This allows urandom requests of even 100k bytes to succeed on a fresh
VM session on a Metro M4 express.

Author: jepler

shared-bindings/os/__init__.c

@@ -33,6 +33,7 @@
 #include "lib/oofatfs/diskio.h"
 #include "py/mpstate.h"
 #include "py/obj.h"
+#include "py/objstr.h"
 #include "py/runtime.h"
 #include "shared-bindings/os/__init__.h"
 
@@ -195,11 +196,11 @@ MP_DEFINE_CONST_FUN_OBJ_0(os_sync_obj, os_sync);
 //|
 STATIC mp_obj_t os_urandom(mp_obj_t size_in) {
     mp_int_t size = mp_obj_get_int(size_in);
-    uint8_t tmp[size];
-    if (!common_hal_os_urandom(tmp, size)) {
+    mp_obj_str_t *result = MP_OBJ_TO_PTR(mp_obj_new_bytes_of_zeros(size));
+    if (!common_hal_os_urandom((uint8_t*) result->data, size)) {
         mp_raise_NotImplementedError(translate("No hardware random available"));
     }
-    return mp_obj_new_bytes(tmp, size);
+    return result;
 }
 MP_DEFINE_CONST_FUN_OBJ_1(os_urandom_obj, os_urandom);