perf buildid: Fix possible unterminated readlink() result buffer

The readlink function doesn't guarantee that a '\0' will be put at the
end of the provided buffer if there is no space left.

No need to do "buf[len] = '\0';" since the buffer is allocated with
zalloc().

Link: http://lkml.kernel.org/r/4E986ABF.9040706@intra2net.comSigned-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/header.c

@@ -1289,7 +1289,7 @@ int build_id_cache__remove_s(const char *sbuild_id, const char *debugdir)
 	if (access(linkname, F_OK))
 		goto out_free;
 
-	if (readlink(linkname, filename, size) < 0)
+	if (readlink(linkname, filename, size - 1) < 0)
 		goto out_free;
 
 	if (unlink(linkname))