Don't print a type of variable in Address::Dump if it's unknown (i.e. nullptr)

Summary: This patch fixes dereferencing of nullptr in case when GetType() returns that.

Reviewers: jingham, granata.enrico, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits, granata.enrico, clayborg, jingham

Differential Revision: http://reviews.llvm.org/D9274

llvm-svn: 235982