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

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

Fixes <rdar://problem/15831804>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199571 91177308-0d34-0410-b5e6-96231b3b80d8