Fix invalid instrument access in xm event reader

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add ST2.6 speed effect processing

According to Saga Musix:

"When both nibbles of an Fxx command are set, SoundTracker 2.6 applies the
 both values alternatingly, first the high nibble, then the low nibble on
 the next row, then the high nibble again... If only the high nibble is set,
 it should act like if only the low nibble is set (i.e. F30 is the same
 as F03)."

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