fix memory corruption when process "*:*" events

In function idmap_clear():
If the id is not set yet, it shouldn't clear idmap and decrement id_nr.

for example:
idmap_set 10, 11, 12
idmap_clear 1
id_nr=2
id_array[0]=i ok
id_array[1]=i ok
id_array[2]=i illegal

The bug can reporduce in 3.2 kernel, because ftrace:function
event is not available in 3.2 kernel.