midi bug remains in 2.13.58

[胡海鹏 - Hu Haipeng]

Hello,
 Lilypond 2.13.58 still has bug on mid output. Because the max number of instr 
is 16, and now the midi output considers every instr instead of track as an 
item, the result of program change still causes error. For example, I have a 
full orch score with 14 sounds plus perc. Then I change strings to pizz, and 
then violin I as solo violin, then trumpet muted. Now there are 18 instr, and 
the topmost instr (picc and fl) changes to string ensemble due to wrapping.

Haipeng