Prevent invalid decrement in tool manager

Iterating over the tool stack, we potentially remove the current
iterator.  If this removal happens at the beginning of the toolstack, we
cannot decrement the iterator to a position before the stack without
creating an invalid state.

common/tool/tool_manager.cpp

@@ -648,16 +648,22 @@ bool TOOL_MANAGER::dispatchInternal( const TOOL_EVENT& aEvent )
 
     wxLogTrace( kicadTraceToolStack, "TOOL_MANAGER::dispatchInternal %s", aEvent.Format() );
 
+    auto it = m_activeTools.begin();
+
     // iterate over active tool stack
-    for( auto it = m_activeTools.begin(); it != m_activeTools.end(); ++it )
+    while( it != m_activeTools.end() )
     {
         TOOL_STATE* st = m_toolIdIndex[*it];
+        bool increment = true;
 
         // forward context menu events to the tool that created the menu
         if( aEvent.IsChoiceMenu() )
         {
             if( *it != m_menuOwner )
+            {
+                ++it;
                 continue;
+            }
         }
 
         // the tool state handler is waiting for events (i.e. called Wait() method)
@@ -685,7 +691,7 @@ bool TOOL_MANAGER::dispatchInternal( const TOOL_EVENT& aEvent )
                     if( end )
                     {
                         it = finishTool( st );
-                        --it;
+                        increment = false;
                     }
                 }
 
@@ -701,6 +707,9 @@ bool TOOL_MANAGER::dispatchInternal( const TOOL_EVENT& aEvent )
                 }
             }
         }
+
+        if( increment )
+            ++it;
     }
 
     for( auto& state : m_toolState )