Fix masking error in InterceptionManager::PatchNtdll

sandbox/win/src/interception.cc

Index: sandbox/win/src/interception.cc
diff --git a/sandbox/win/src/interception.cc b/sandbox/win/src/interception.cc
index dde585735f13b763347ecc53f96bba70145f9ffe..597b937bbe77f5d3495be03b4f5c3d60704254ca 100644
--- a/sandbox/win/src/interception.cc
+++ b/sandbox/win/src/interception.cc
@@ -399,7 +399,7 @@ bool InterceptionManager::PatchNtdll(bool hot_patch_needed) {
   thunk_offset &= kPageSize - 1;
 
   // Make an aligned, padded allocation, and move the pointer to our chunk.
-  size_t thunk_bytes_padded = (thunk_bytes + kPageSize - 1) & kPageSize;
+  size_t thunk_bytes_padded = (thunk_bytes + kPageSize - 1) & ~(kPageSize - 1);
   thunk_base = reinterpret_cast<BYTE*>(
                    ::VirtualAllocEx(child, thunk_base, thunk_bytes_padded,
                                     MEM_COMMIT, PAGE_EXECUTE_READWRITE));