Fix segfault when doing `thread info` on a thread without stop info.

Summary:
E.g., if thread 1 hits a breakpoint, then a `thread info` on thread 2 will cause
a segfault, since thread 2 will have no stop info (intended behavior?).

Reviewers: kubabrecka, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8905

llvm-svn: 234437