SecurityPkg/DxeImageVerificationHandler: fix retval for (FileBuffer==NULL)

"FileBuffer" is a non-optional input (pointer) parameter to
DxeImageVerificationHandler(). Normally, when an edk2 function receives a
NULL argument for such a parameter, we return EFI_INVALID_PARAMETER or
RETURN_INVALID_PARAMETER. However, those don't conform to the
SECURITY2_FILE_AUTHENTICATION_HANDLER prototype.

Return EFI_ACCESS_DENIED when "FileBuffer" is NULL; it means that no image
has been loaded.

This patch does not change the control flow in the function, it only
changes the "Status" outcome from API-incompatible error codes to
EFI_ACCESS_DENIED, under some circumstances.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2129
Fixes: 570b3d1a72
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200116190705.18816-10-lersek@redhat.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
[lersek@redhat.com: push with Mike's R-b due to Chinese New Year
 Holiday: <https://edk2.groups.io/g/devel/message/53429>; msgid
 <d3fbb76dabed4e1987c512c328c82810@intel.com>]

SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c

@@ -1655,7 +1655,7 @@ DxeImageVerificationHandler (
   // Read the Dos header.
   //
   if (FileBuffer == NULL) {
-    return EFI_INVALID_PARAMETER;
+    return EFI_ACCESS_DENIED;
   }
 
   mImageBase  = (UINT8 *) FileBuffer;