[PATCH] MdePkg/BasePrintLib: Fix incorrect Precision position calculation

[Jian J Wang <jian.j.wang@intel.com>]

Due to the a potential hole in the stop condition of for-loop, the two
continuous access to ArgumentString (index, index+1) inside the loop
might cause the string ending character ('\0') to be read.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
---
 MdePkg/Library/BasePrintLib/PrintLibInternal.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/MdePkg/Library/BasePrintLib/PrintLibInternal.c b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
index 28d946472f..297d5a05b5 100644
--- a/MdePkg/Library/BasePrintLib/PrintLibInternal.c
+++ b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
@@ -1107,7 +1107,10 @@ BasePrintLibSPrintMarker (
       // Compute the number of characters in ArgumentString and store it in Count
       // ArgumentString is either null-terminated, or it contains Precision characters
       //
-      for (Count = 0; Count < Precision || ((Flags & PRECISION) == 0); Count++) {
+      for (Count = 0;
+            ArgumentString[Count * BytesPerArgumentCharacter] != '\0' &&
+            (Count < Precision || ((Flags & PRECISION) == 0));
+              Count++) {
         ArgumentCharacter = ((ArgumentString[Count * BytesPerArgumentCharacter] & 0xff) | ((ArgumentString[Count * BytesPerArgumentCharacter + 1]) << 8)) & ArgumentMask;
         if (ArgumentCharacter == 0) {
           break;
-- 
2.15.1.windows.2