perf script: Remove some bogus error handling

If script_desc__new() fails then the current code has a NULL
dereference.  We don't actually need to do any cleanup, we can just
return NULL.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel-janitors@vger.kernel.org
Link: http://lkml.kernel.org/r/20170722073610.nnsyiwdcfl6bhn4t@mwanda


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>