Fix single-stepping onto a breakpoint

Summary:
r259344 introduced a bug, where we fail to perform a single step, when the instruction we are
stepping onto contains a breakpoint which is not valid for this thread. This fixes the problem
and add a test case.

Reviewers: tberghammer, emaste

Subscribers: abhishek.aggarwal, lldb-commits, emaste

Differential Revision: http://reviews.llvm.org/D16767

llvm-svn: 259488