How to reproduce:
1. Open the attached "New Unity Project 4" project and open "1" scene
2. Select two or more "Cube" prefabs from Assets window
3. Drag and drop them onto or between GameObjects in Hierarchy while holding ALT key

Expected result: Editor doesn't crash
Actual result: Editor crashes

Reproducible with - 2017.1.0a3, 5.6.0f1, 5.5.2p3, 5.4.5f1

Notes:
1. In 5.6 or 2017.1 versions, Editor won't crash if you drag and drop Prefabs between GameObjects, but will crash if you will drag and drop Prefabs onto GameObject
2. Crash might not occur the first time, but usually happens around second or third time