tsan: clear shadow for ucontext, because it's visible to user


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@159365 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/tsan/rtl/tsan_interceptors.cc b/lib/tsan/rtl/tsan_interceptors.cc
index e6d323a..27eb35d 100644
--- a/lib/tsan/rtl/tsan_interceptors.cc
+++ b/lib/tsan/rtl/tsan_interceptors.cc
@@ -117,6 +117,7 @@
     ScopedInRtl in_rtl;
     ctx = (SignalContext*)internal_alloc(
         MBlockSignal, sizeof(*ctx));
+    MemoryResetRange(thr, 0, (uptr)ctx, sizeof(*ctx));
     internal_memset(ctx, 0, sizeof(*ctx));
     thr->signal_ctx = ctx;
   }