Fix a use after free I introduced and that Bill caught in code review
(thanks!) by deferring the free of the filename until we finish writing
the coverage data to that file.

Bill, let me know if you'd prefer a different approach!

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@184895 91177308-0d34-0410-b5e6-96231b3b80d8