[dasher] Fix deleting after having traversed control nodes.

[Patrick Welche <pwelche src gnome org>]

commit 9d996a52a26f403debec245c018970572b906e7e
Author: Alan Lawrence <acl33 inf phy cam ac uk>
Date:   Thu May 12 13:04:52 2011 +0100

    Fix deleting after having traversed control nodes.
    
    string::substr(start,len) allows for len to take you beyond the end of
    the string - in which case, you just get back the rest of the string -
    but not for start to be before the beginning.

 Src/DasherCore/AlphabetManager.cpp |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/Src/DasherCore/AlphabetManager.cpp b/Src/DasherCore/AlphabetManager.cpp
index f123879..8422c23 100644
--- a/Src/DasherCore/AlphabetManager.cpp
+++ b/Src/DasherCore/AlphabetManager.cpp
@@ -567,7 +567,8 @@ void CAlphabetManager::CSymbolNode::Undo(int *pNumDeleted) {
       // iff this node was actually written (i.e. not rebuilt _from_ context!)
       std::string &buf(m_pMgr->strTrainfileBuffer);
       std::string tr(trainText());
-      if (buf.substr(buf.length()-tr.length(),tr.length())==tr) {
+      if (tr.length()<=buf.length()
+          && buf.substr(buf.length()-tr.length(),tr.length())==tr) {
         buf=buf.substr(0,buf.length()-tr.length());
         m_pMgr->m_pLastOutput = Parent();
       }