Fix parameters for CreateLowBoxToken

When calling CreateLowBoxToken, the kernel API will throw an error if
there is a mismatch between the saved_handles_count and the
saved_handles array. If the count is 0, the pointer must be null.

This change passes in nullptr to CreateLowBoxToken for saved_handles if
saved_handles_count is 0.

Bug: 1000447
Change-Id: I2e5706944e5348ae1833eb28562945b7874f6a75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1845831
Reviewed-by: James Forshaw <forshaw@chromium.org>
Commit-Queue: Cliff Smolinsky <cliffsmo@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#703378}

sandbox/win/src/restricted_token_utils.cc

@@ -368,7 +368,8 @@ DWORD CreateLowBoxToken(HANDLE base_token,
       &token_lowbox, base_token, TOKEN_ALL_ACCESS, &obj_attr,
       security_capabilities->AppContainerSid,
       security_capabilities->CapabilityCount,
-      security_capabilities->Capabilities, saved_handles_count, saved_handles);
+      security_capabilities->Capabilities, saved_handles_count,
+      saved_handles_count > 0 ? saved_handles : nullptr);
   if (!NT_SUCCESS(status))
     return GetLastErrorFromNtStatus(status);