Don't have ExecutionContextRef::SetTargetPtr fill in the frame

information if we're not stopped.  This could try to read registers
etc when the process is still running and debug builds of lldb would
assert down in GDBRemoteRegisterContext.cpp ReadRegisterBytes because
we couldn't get the sequence mutex for talking to the remote system.
Non-debug builds would just silently fail when doing this.
<rdar://problem/11941758>

llvm-svn: 160829