Fix RGBHandler::canRead

Summary:
As one can see in SGIImage::readImage the accepted images are

_stream >> u16;
if (u16 != 0x01da) {
    return false;
}

_stream >> _rle;
if (_rle > 1) {
    return false;
}

so not only \x01\xda\x01 but also \x01\xda\x00

Reviewers: svuorela

Reviewed By: svuorela

Subscribers: svuorela, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20145