[libFuzzer] fix the bot (the no-assert build)

llvm-svn: 337437
diff --git a/compiler-rt/lib/fuzzer/FuzzerMutate.cpp b/compiler-rt/lib/fuzzer/FuzzerMutate.cpp
index 9c2b8a5..ff076cc 100644
--- a/compiler-rt/lib/fuzzer/FuzzerMutate.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerMutate.cpp
@@ -549,6 +549,7 @@
   assert(!T.empty());
   size_t NewSize = Mutate(T.data(), OneBits, OneBits);
   assert(NewSize <= OneBits);
+  (void)NewSize;
   // Even if NewSize < OneBits we still use all OneBits bytes.
   for (size_t I = 0, J = 0; I < Size; I++)
     if (Mask[I])