perf tools: Fix uninitialized pathname on uncompressed fallback in filename__decompress()

perf tools: Fix uninitialized pathname on uncompressed fallback in filename__decompress()

filename__decompress() has an early return path for files that are not
actually compressed.  This path returns the fd from open() directly but
never writes to the pathname output parameter, leaving the caller with
an uninitialized buffer despite a successful return.

Callers like dso__decompress_kmodule_path() pass pathname to
decompress_kmodule() which uses it to set the decompressed file path.
If pathname is uninitialized, subsequent operations on the path produce
undefined behavior.

Fix by setting pathname to an empty string on the uncompressed path.
Callers already check for an empty pathname to distinguish temporary
decompressed files (which need unlink) from the original file.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Fixes: 7ac22b088afe26a4 ("perf tools: Add filename__decompress function")
Cc: Jiri Olsa <jolsa@kernel.org>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>