Editline: Fix an msan error

Summary:
libedit implementation of el_get(EL_GETTC) had a bug, where it was
consuming vararg arguments until reaching the first null pointer (and
not just two, as documented). This was causing (at least) errors to be
reported when running the tests under msan.

The issue has since been fixed in libedit, but this adds patch adds a
trivial workaround, so that we operate correctly with the libedit
versions which are already out there.

Reviewers: christos, krytarowski, davide

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D61191

llvm-svn: 359449