lldb - fix misleading "valid target indexes are" error message

[pawelo]

Summary

This fixes following issue:

(lldb) target select 0
error: index 0 is out of range, valid target indexes are 0 - 4294967295
(lldb) target select 1
error: index 1 is out of range, valid target indexes are 0 - 4294967295

Due to unsigned arithmetic operation, we can see above output which is both misleading and against pure logic.

The question why there are no targets listed remains open.

[tfiala]

See comments here, Paul:
http://lists.cs.uiuc.edu/pipermail/lldb-commits/Week-of-Mon-20140825/012666.html

I'll wait to see how you adjust this per comments, then get it in.

[pawelo]

Improved error message.

[tfiala]

svn commit
Sending source/Commands/CommandObjectTarget.cpp
Transmitting file data .
Committed revision 216766.

source/Commands/CommandObjectTarget.cpp

DoExecute (Args& args, CommandReturnObject &result)
                     else
                     {
                         result.AppendErrorWithFormat ("target #%u is NULL in target list\n", target_idx);
                         result.SetStatus (eReturnStatusFailed);
                     }
                 }
                 else
                 {
-                    result.AppendErrorWithFormat ("index %u is out of range, valid target indexes are 0 - %u\n", 
+                    if (num_targets > 0)
+                    {
+                        result.AppendErrorWithFormat ("index %u is out of range, valid target indexes are 0 - %u\n",
                                                       target_idx,
                                                       num_targets - 1);
+                    } else
+                    {
+                        result.AppendErrorWithFormat ("index %u is out of range since there are no active targets\n",
+                                                      target_idx);
+                    }
                     result.SetStatus (eReturnStatusFailed);
                 }
             }
             else
             {
                 result.AppendErrorWithFormat("invalid index string value '%s'\n", target_idx_arg);
                 result.SetStatus (eReturnStatusFailed);
             }