x86/hyperv: Fix error pointer dereference

x86/hyperv: Fix error pointer dereference

The function idle_thread_get() can return an error pointer and is not
checked for it. Add check for error pointer.

Detected by Smatch:
arch/x86/hyperv/hv_vtl.c:126 hv_vtl_bringup_vcpu() error:
'idle' dereferencing possible ERR_PTR()

Fixes: 2b4b90e053a29 ("x86/hyperv: Use per cpu initial stack for vtl context")
Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>