[patch] fix bug 3974: crash with user defined external templates

[Bernhard Roider]

Hello,

the crash occured because ControlExternal::getTemplate() is called with the index -1 which produces 
an invalid iterator that is accessed afterwards.

Bernhard
Index: src/frontends/controllers/ControlExternal.cpp
===================================================================
--- src/frontends/controllers/ControlExternal.cpp       (revision 18994)
+++ src/frontends/controllers/ControlExternal.cpp       (working copy)
@@ -135,7 +135,9 @@
        external::TemplateManager::Templates::const_iterator i1
                = external::TemplateManager::get().getTemplates().begin();
 
-       advance(i1, i);
+       if (i >= 0) {
+               advance(i1, i);
+       }
 
        return i1->second;
 }