GLOBAL MARKETS-Geopolitical tensions spark major move to safer assets

* Crash of Malaysian airliner sparks risk-aversion * Russian assets fall on U.S., EU sanctions * U.S. housing starts weak; Morgan Stanley results up (Updates to the close of U.S. trading, adds Gaza offensive) By Ryan Vlastelica NEW YORK, July 17 (Reuters) - Geopolitical tensions weighed heavily on global stock markets on Thursday, with indexes extending their declines in afternoon trading while safe-haven assets rose. Wall Street stocks started the day with modest losses af