UefiCpuPkg/MpInitLibUp: Fix #UD exception due to NULL pointer deref

In MpInitLibUp, MpInitLibGetNumberOfProcessors() unconditionally
dereferences the NumberOfProcessors and NumberOfEnabledProcessors
parameters. However, the API specification marks both as OPTIONAL.

When callers (like InitializeExceptionStackSwitchHandlers in
UefiCpuPkg/CpuMpPei/CpuMpPei.c) pass NULL for NumberOfEnabledProcessors,
GCC's Link-Time Optimization (LTO) detects an unconditional NULL pointer
dereference. Since this is Undefined Behavior, GCC emits a 'ud2'
(Invalid Opcode) instruction at the dereference site. This leads to an
unexpected #UD exception during the boot process of uniprocessor guests
like TDX VMs.

This patch fixes the issue by adding appropriate NULL checks before
dereferencing the pointers. It also returns EFI_INVALID_PARAMETER if
both arguments are NULL, ensuring compliance with the MpInitLib
specification.

Signed-off-by: Changyuan Lyu <changyuanl@google.com>

UefiCpuPkg/Library/MpInitLibUp/MpInitLibUp.c

@@ -68,8 +68,18 @@ MpInitLibGetNumberOfProcessors (
   OUT UINTN  *NumberOfEnabledProcessors OPTIONAL
   )
 {
-  *NumberOfProcessors        = 1;
-  *NumberOfEnabledProcessors = 1;
+  if ((NumberOfProcessors == NULL) && (NumberOfEnabledProcessors == NULL)) {
+    return EFI_INVALID_PARAMETER;
+  }
+
+  if (NumberOfProcessors != NULL) {
+    *NumberOfProcessors = 1;
+  }
+
+  if (NumberOfEnabledProcessors != NULL) {
+    *NumberOfEnabledProcessors = 1;
+  }
+
   return EFI_SUCCESS;
 }