Fixes a stack-use-after-scope sanitizer issue

We kept a const referenced copy of the sink file in the SinkFile class, Change
it to just keep a copy.

Bug: crbug.com/849192
Test: USE="asan fuzzer" emerge-amd64-generic bsdiff && ASAN_OPTIONS=log_path=stderr /build/amd64-generic/usr/libexec/fuzzers/bspatch_fuzzer ./clusterfuzz-testcase-minimized-bspatch_fuzzer-5673050675347456
Change-Id: Ia583620d337a780c202cdb07233317ecb3fe26c8
diff --git a/sink_file.h b/sink_file.h
index e2c69d8..ebec522 100644
--- a/sink_file.h
+++ b/sink_file.h
@@ -33,7 +33,7 @@
 
  private:
   // The sink() function used to write data.
-  const sink_func& sink_;
+  const sink_func sink_;
 };
 
 }  // namespace bsdiff