[libFuzzer] fix missing close on opened file

Summary:
When running the standalone main on a large corpus, I eventually get a
EMFILE error ("Too many open files").

Patch by Paul Chaignon

Reviewers: kcc, vitalybuka

Reviewed By: vitalybuka

Subscribers: lebedev.ri, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D58622

llvm-svn: 354918