perfcounters/powerpc: fix oops with multiple counters in a group

Impact: fix oops-causing bug

This fixes a bug in the powerpc hw_perf_counter_init where the code
didn't initialize ctrs[n] before passing the ctrs array to check_excludes,
leading to possible oopses and other incorrect behaviour.  This fixes it
by initializing ctrs[n] correctly.

Signed-off-by: Paul Mackerras <paulus@samba.org>