Fix XM portamento target reset on new instrument

According to the OpenMPT Porta-Pickup.xm test case:

"An instrument number should not reset the current portamento target. The
 portamento target is valid until a new target is specified by combining a
 note and a portamento effect."

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT Porta-Pickup.xm test case

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>