perf test sdt: Handle realpath() failure

It can return NULL, in which case we should bail out and remove the
directory created with mkdtemp(), which is stored in the "__tempdir"
variable, not in "tempdir".

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 8e5dc848 ("perf test: Add a test case for SDT event")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>