protect libedit and LLDB gui from receiving null FILE* streams

Summary:
We now have valid files that will return NULL from GetStream().
libedit and the LLDB gui are the only places left that need FILE*
streams.  Both are doing curses-like user interaction that only
make sense with a real terminal anyway, so there is no need to convert
them off of their use of FILE*.   But we should check for null streams
before enabling these features.

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: JDevlieghere, labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68677

llvm-svn: 374197