Pull in r199571 from upstream clang trunk (by Ted Kremenek):

Harden InitListExpr::isStringLiteralInit() against getInit()
returning null.

This led to a crash on invalid code (sorry, no good test case).

Fixes <rdar://problem/15831804>.

This fixes an assertion when compiling certain incorrect code, as
reported upstream in http://llvm.org/PR22684 .

Direct commit to stable/10 and stable/9, since head has clang 3.5.1,
which already includes this change.

Reported by: hbowden@securelabsllc.com