[PATCH v2] MdeModulePkg/SdDxe: Fix potential NULL pointer access

[Jeff Brasen <jbrasen@nvidia.com>]

SdReadWrite can be called with a NULL Token for synchronous operations.
Add guard for DEBUG print to only print event pointer with Token is not
NULL.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jeff Brasen <jbrasen@nvidia.com>
---
 MdeModulePkg/Bus/Sd/SdDxe/SdBlockIo.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/MdeModulePkg/Bus/Sd/SdDxe/SdBlockIo.c b/MdeModulePkg/Bus/Sd/SdDxe/SdBlockIo.c
index b8d115a..a4695ff 100644
--- a/MdeModulePkg/Bus/Sd/SdDxe/SdBlockIo.c
+++ b/MdeModulePkg/Bus/Sd/SdDxe/SdBlockIo.c
@@ -670,8 +670,9 @@ SdReadWrite (
     if (EFI_ERROR (Status)) {
       return Status;
     }
-    DEBUG ((DEBUG_BLKIO, "Sd%a(): Lba 0x%x BlkNo 0x%x Event %p with %r\n", IsRead ? "Read" : "Write", Lba, BlockNum, Token->Event, Status));
-
+    DEBUG ((DEBUG_BLKIO, "Sd%a(): Lba 0x%x BlkNo 0x%x Event %p with %r\n",
+      IsRead ? "Read" : "Write", Lba, BlockNum,
+      (Token != NULL) ? Token->Event : NULL, Status));
     Lba   += BlockNum;
     Buffer = (UINT8*)Buffer + BufferSize;
     Remaining -= BlockNum;
-- 
2.7.4