objexcept: Optimize traceback allocation for exception.

Traceback allocation for exception will now never lead to recursive
MemoryError exception - if there's no memory for traceback, it simply
won't be created.

py/objexcept.c

@@ -436,11 +436,15 @@ void mp_obj_exception_add_traceback(mp_obj_t self_in, qstr file, mp_uint_t line,
 
     // for traceback, we are just using the list object for convenience, it's not really a list of Python objects
     if (self->traceback == MP_OBJ_NULL) {
-        self->traceback = mp_obj_new_list(0, NULL);
+        self->traceback = mp_obj_new_list_maybe(3);
+        if (self->traceback == MP_OBJ_NULL) {
+            return;
+        }
     }
-    mp_obj_list_append(self->traceback, (mp_obj_t)(mp_uint_t)file);
-    mp_obj_list_append(self->traceback, (mp_obj_t)(mp_uint_t)line);
-    mp_obj_list_append(self->traceback, (mp_obj_t)(mp_uint_t)block);
+    mp_obj_list_t *list = self->traceback;
+    list->items[0] = (mp_obj_t)(mp_uint_t)file;
+    list->items[1] = (mp_obj_t)(mp_uint_t)line;
+    list->items[2] = (mp_obj_t)(mp_uint_t)block;
 }
 
 void mp_obj_exception_get_traceback(mp_obj_t self_in, mp_uint_t *n, mp_uint_t **values) {

py/objlist.c

@@ -477,6 +477,25 @@ mp_obj_t mp_obj_new_list(mp_uint_t n, mp_obj_t *items) {
     return o;
 }
 
+// Special method for usage with exceptions
+// Doesn't initialize items, assumes they will be initialized by client.
+mp_obj_t mp_obj_new_list_maybe(mp_uint_t n) {
+    mp_obj_list_t *o = m_new_obj_maybe(mp_obj_list_t);
+    if (!o) {
+        return o;
+    }
+    o->items = m_new_maybe(mp_obj_t, n);
+    if (!o->items) {
+        m_del_obj(mp_obj_list_t, o);
+        return MP_OBJ_NULL;
+    }
+
+    o->base.type = &mp_type_list;
+    o->len = o->alloc = n;
+
+    return o;
+}
+
 void mp_obj_list_get(mp_obj_t self_in, mp_uint_t *len, mp_obj_t **items) {
     mp_obj_list_t *self = self_in;
     *len = self->len;

py/objlist.h

@@ -35,4 +35,6 @@ typedef struct _mp_obj_list_t {
     mp_obj_t *items;
 } mp_obj_list_t;
 
+mp_obj_t mp_obj_new_list_maybe(mp_uint_t n);
+
 #endif // __MICROPY_INCLUDED_PY_OBJLIST_H__