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