Check the result of creating a node from __next_ in the std::list formatter.

There's a single report of a crash coming from this current_sp being NULL.  I don't
have a repro case, and I couldn't get it to happen by hand-corrupting a list.  We
always get an error instead.  So I don't have a test case.  But checking for null
is clearly right here.

<rdar://problem/48503320>

llvm-svn: 355957