Remove side-effect from finishTool().

It belongs out in the calling loop so that if someone else uses
finishTool() later they won't get bit by it.

This also fixes a crash bug where we weren't deactivating all tools
because we couldn't back up at the beginning of the stack (and the
loop increment therefore went past the first element).

Fixes https://gitlab.com/kicad/code/kicad/issues/4206

common/tool/tool_manager.cpp

@@ -496,7 +496,7 @@ void TOOL_MANAGER::ShutdownTool( TOOL_BASE* aTool )
                 bool end = !st->cofunc->Resume();
 
                 if( end )
-                    it = finishTool( st );
+                    finishTool( st );
             }
         }
     }
@@ -683,7 +683,10 @@ bool TOOL_MANAGER::dispatchInternal( const TOOL_EVENT& aEvent )
                     bool end = !st->cofunc->Resume();
 
                     if( end )
+                    {
                         it = finishTool( st );
+                        --it;
+                    }
                 }
 
                 // If the tool did not request the event be passed to other tools, we're done
@@ -918,11 +921,7 @@ TOOL_MANAGER::ID_LIST::iterator TOOL_MANAGER::finishTool( TOOL_STATE* aState )
     if( tool->GetType() == INTERACTIVE )
         static_cast<TOOL_INTERACTIVE*>( tool )->resetTransitions();
 
-    // Don't move the iterator past the stack beginning
-    if( it == m_activeTools.begin() )
-        return it;
-
-    return --it;
+    return it;
 }