perf trace scripting: fix some small memory leaks and missing error checks

Free the other two fields of script_desc which somehow got overlooked,
free malloc'ed args in case exec fails, and add missing checks for
failed mallocs.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Acked-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>