Default value of pcc_subspace_idx is -1.
Make sure to check pcc_subspace_idx before using the same as array index.
This will avoid following KASAN warnings too.

[   15.113449] ==================================================================
[   15.116983] BUG: KASAN: global-out-of-bounds in cppc_get_perf_caps+0xf3/0x3b0
[   15.116983] Read of size 8 at addr ffffffffb9a5c0d8 by task swapper/0/1
[   15.116983] CPU: 3 PID: 1 Comm: swapper/0 Not tainted 4.15.0-rc2+ #2
[   15.116983] Hardware name: Dell Inc. OptiPlex 7040/0Y7WYT, BIOS 1.2.8 01/26/2016
[   15.116983] Call Trace:
[   15.116983]  dump_stack+0x7c/0xbb
[   15.116983]  print_address_description+0x1df/0x290
[   15.116983]  kasan_report+0x28a/0x370
[   15.116983]  ? cppc_get_perf_caps+0xf3/0x3b0
[   15.116983]  cppc_get_perf_caps+0xf3/0x3b0
[   15.116983]  ? cpc_read+0x210/0x210
[   15.116983]  ? __rdmsr_on_cpu+0x90/0x90
[   15.116983]  ? rdmsrl_on_cpu+0xa9/0xe0
[   15.116983]  ? rdmsr_on_cpu+0x10...