Initializer::mirror_keyframe_map init_frame_index_last might cause crash

[kismeter]

Hi

`void Initializer::mirror_keyframe_map(Map *feature_tracking_map,
size_t init_frame_id) {
size_t init_frame_index_last =
feature_tracking_map->frame_index_by_id(init_frame_id);
size_t init_frame_index_gap = config->initializer_keyframe_gap();
size_t init_frame_index_distance =
init_frame_index_gap * (config->initializer_keyframe_num() - 1);

...
}`

in the Initializer.cpp, in some cases above code the value init_frame_index_last could be nil(), it's size_t(-1), the value is not handle, that could cause out of bound