perf script: Fix mem leaks and NULL pointer checks around strdup()s

Fix mem leaks and missing NULL pointer checks after strdup().

And get_script_path() did not free __script_root in case of continue.

Introduce a helper function get_script_root().

Cc: Ingo Molnar <mingo@elte.hu>
Link: http://lkml.kernel.org/r/1322217520-3287-1-git-send-email-robert.richter@amd.com
Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>