Check for valid shared pointer before locking

We don't neccesarily have a valid pointer to the parent lib symbol when
checking if it is a power symbol, so fall back to the local flag if this
is the case

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17182

(cherry picked from commit 4481fcc75e)

eeschema/lib_symbol.cpp

@@ -653,7 +653,9 @@ const wxString LIB_SYMBOL::GetLibraryName() const
 
 bool LIB_SYMBOL::IsPower() const
 {
-    if( LIB_SYMBOL_SPTR parent = m_parent.lock() )
+    std::shared_ptr<LIB_SYMBOL> parent;
+
+    if( !m_parent.expired() && ( parent = m_parent.lock() ) )
     {
         if( parent->IsRoot() )
             return parent->m_options == ENTRY_POWER;