Fix disabled 'save library' button in the symbol editor

With uninitialized COMPONENT_TREE::m_menuActive there were cases when
application logic erroneously decided to fetch the current library name
from the right click menu selection, even though there was no menu
opened, so an empty string was returned. Empty library name is
equivalent to no library loaded, therefore the save button appeared
disabled.

Fixes: lp:1765937
* https://bugs.launchpad.net/kicad/+bug/1765937

eeschema/widgets/component_tree.cpp

@@ -41,6 +41,7 @@ COMPONENT_TREE::COMPONENT_TREE( wxWindow* aParent, SYMBOL_LIB_TABLE* aSymLibTabl
       m_adapter( aAdapter ),
       m_query_ctrl( nullptr ),
       m_details_ctrl( nullptr ),
+      m_menuActive( false ),
       m_filtering( false )
 {
     // create space for context menu pointers, INVALID is the max value