swift234

[Driver] Fix an uninitialized variable.

This only shows up when there's a present-but-empty sequence node in
a swiftdeps file. The compiler never generates this, but it's in some of
the tests. I'm not sure why ASan didn't catch this.

This should fix the failures some people have been seeing with
test/Driver/Dependencies/private-after.swift, rdar://problem/23382722.