Fix possible memory leak in error path when elf notes are backed by a memfd path

[dalehamel]

As @palmtenor points out, #2314 (comment), the original PR misses freeing the memory that it allocates for a string on the error path (where the directory listing fails) resulting in a potential memory leak.

I had considered just using a separate buffer for the directory string, but decided to malloc it so I could reuse the buffer for building the full path while still retaining access to the string. If there's a reason to prefer that approach I can modify this PR according, which would eliminate the need to bother with freeing this string buffer as it would be stack-allocated instead.

[palmtenor]

[buildbot, ok to test]

[yonghong-song]

[buildbot, test this please]