eeschema: reject empty schematic tables during parse

Empty table nodes from malformed input can leave table state invalid and
crash later accesses.

Found by codex-5.3

eeschema/sch_io/kicad_sexpr/sch_io_kicad_sexpr_parser.cpp

@@ -5037,6 +5037,12 @@ SCH_TABLE* SCH_IO_KICAD_SEXPR_PARSER::parseSchTable()
         }
     }
 
+    if( !table->GetCell( 0, 0 ) )
+    {
+        THROW_PARSE_ERROR( _( "Invalid table: no cells defined" ), CurSource(), CurLine(), CurLineNumber(),
+                           CurOffset() );
+    }
+
     return table.release();
 }
 

eeschema/sch_table.cpp

@@ -261,7 +261,9 @@ void SCH_TABLE::RunOnChildren( const std::function<void( SCH_ITEM* )>& aFunction
 
 const BOX2I SCH_TABLE::GetBoundingBox() const
 {
-    // Note: a table with no cells is not allowed
+    if( m_cells.empty() )
+        return BOX2I();
+
     BOX2I bbox = m_cells[0]->GetBoundingBox();
 
     bbox.Merge( m_cells[m_cells.size() - 1]->GetBoundingBox() );