Update [fl_accessible_text_field_get_text](https://github.com/flutter/flutter/blob/be21b93d5506bd67bc927df686545aadc96056a2/engine/src/flutter/shell/platform/linux/fl_accessible_text_field.cc#L306-L314) and [fl_accessible_text_field_get_text_at_offset](https://github.com/flutter/flutter/blob/be21b93d5506bd67bc927df686545aadc96056a2/engine/src/flutter/shell/platform/linux/fl_accessible_text_field.cc#L342-L373) to prevent out-of-bounds memory access when ATK clients pass offsets beyond the text length.
Update fl_accessible_text_field_get_text and fl_accessible_text_field_get_text_at_offset to prevent out-of-bounds memory access when ATK clients pass offsets beyond the text length.