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

[chaoren]

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?).

[clayborg]

Looks good.

lldb/trunk/source/Target/Thread.cpp

 
 bool
 Thread::GetDescription (Stream &strm, lldb::DescriptionLevel level, bool print_json_thread, bool print_json_stopinfo)
 {
     DumpUsingSettingsFormat (strm, 0);
     strm.Printf("\n");
 
     StructuredData::ObjectSP thread_info = GetExtendedInfo();
-    StructuredData::ObjectSP stop_info = m_stop_info_sp->GetExtendedInfo();
-    
     if (print_json_thread || print_json_stopinfo)
     {
         if (thread_info && print_json_thread)
         {
             thread_info->Dump (strm);
             strm.Printf("\n");
         }
-        
-        if (stop_info && print_json_stopinfo)
+        if (print_json_stopinfo && m_stop_info_sp)
+        {
+            StructuredData::ObjectSP stop_info = m_stop_info_sp->GetExtendedInfo();
+            if (stop_info)
             {
                 stop_info->Dump (strm);
                 strm.Printf("\n");
             }
-        
+        }
         return true;
     }
 
     if (thread_info)
     {
         StructuredData::ObjectSP activity = thread_info->GetObjectForDotSeparatedPath("activity");
         StructuredData::ObjectSP breadcrumb = thread_info->GetObjectForDotSeparatedPath("breadcrumb");
         StructuredData::ObjectSP messages = thread_info->GetObjectForDotSeparatedPath("trace_messages");