x86/hyperv: Fix NULL deref in set_hv_tscchange_cb() if Hyper-V setup fails

In the Linux kernel, the following vulnerability has been resolved:

x86/hyperv: Fix NULL deref in set_hv_tscchange_cb() if Hyper-V setup fails

Check for a valid hv_vp_index array prior to derefencing hv_vp_index when
setting Hyper-V's TSC change callback. If Hyper-V setup failed in
hyperv_init(), the kernel will still report that it's running under
Hyper-V, but will have silently disabled nearly all functionality.

BUG: kernel NULL pointer dereference, address: 0000000000000010
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] SMP
CPU: 4 PID: 1 Comm: swapper/0 Not tainted 5.15.0-rc2+ #75
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:set_hv_tscchange_cb+0x15/0xa0
Code: <8b> 04 82 8b 15 12 17 85 01 48 c1 e0 20 48 0d ee 00 01 00 f6 c6 08
...
Call Trace:
kvm_arch_init+0x17c/0x280
kvm_init+0x31/0x330
vmx_init+0xba/0x13a
do_one_initcall+0x41/0x1c0
kernel_init_freeable+0x1f2/0x23b
kernel_init+0x16/0x120
ret_from_fork+0x22/0x30