Fix error recovery with in-class initializer.

Previously, for a field with an invalid in-class initializer, we
would create a CXXDefaultInitExpr referring to a null Expr*.
This is not a good idea.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185216 91177308-0d34-0410-b5e6-96231b3b80d8