x86/hvmloader: fix usage of NULL with cpuid_count()

x86/hvmloader: fix usage of NULL with cpuid_count()

The commit that added support for retrieving the APIC IDs from the APs
introduced several usages of cpuid() with NULL parameters, which is not
handled by the underlying implementation.  For GCC I expect this results in
writes to the physical address at 0, however when using Clang 19.1.2 the
generated code in smp.o for the whole file is:

tools/firmware/hvmloader/smp.o: file format elf32-i386

Disassembly of section .text:

00000000 <smp_initialise>:
       0: 55                            pushl   %ebp
       1: 89 e5                         movl    %esp, %ebp
       3: 53                            pushl   %ebx
       4: 31 c0                         xorl    %eax, %eax
       6: 31 c9                         xorl    %ecx, %ecx
       8: 0f a2                         cpuid

Showing the usage of a NULL pointer results in undefined behavior, and
Clang refusing to generate further code after it.

Fix by using a temporary variable in cpuid_count() in place for any NULL
parameter.

Fixes: 9ad0db58c7e2 ('tools/hvmloader: Retrieve APIC IDs from the APs themselves')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>