Add initializer for statistics field (#2038)

Copying uninitialized pointers is undefined behavior, and security
mitigations such as structure protection [1] take advantage of this.
Previously benchmarks would crash when copying the uninitialized
statistics field; fix the crash by initializing it.

[1] https://discourse.llvm.org/t/rfc-structure-protection-a-family-of-uaf-mitigation-techniques/85555
diff --git a/include/benchmark/benchmark.h b/include/benchmark/benchmark.h
index f88f648..3d83a4d 100644
--- a/include/benchmark/benchmark.h
+++ b/include/benchmark/benchmark.h
@@ -1821,6 +1821,7 @@
           complexity(oNone),
           complexity_lambda(),
           complexity_n(0),
+          statistics(),
           report_big_o(false),
           report_rms(false),
           allocs_per_iter(0.0) {}