Fix and integer overflow issue in SDK's QuickSort

BUG=382639
R=mdempsky@chromium.org

Review URL: https://codereview.chromium.org/354673002

fpdfsdk/include/fsdk_mgr.h

diff --git a/fpdfsdk/include/fsdk_mgr.h b/fpdfsdk/include/fsdk_mgr.h
index bb25b35..e6a9219 100644
--- a/fpdfsdk/include/fsdk_mgr.h
+++ b/fpdfsdk/include/fsdk_mgr.h
@@ -564,7 +564,7 @@
 			return;
 		}
 		
-		FX_UINT m = (nStartPos + nStopPos) / 2;
+		FX_UINT m = nStartPos + (nStopPos - nStartPos) / 2;
 		FX_UINT i = nStartPos;
 		
 		TYPE Value = this->GetAt(m);