[COVERITY] Fix memory leak in libss (ss_execute_line)

[COVERITY] Fix memory leak in libss (ss_execute_line)

Fix a memory leak by freeing the argv[] array if ss_parse_line returns 0
for argc 0 (which will happen if the user his return and sends an empty
line to the application).

Potentially need to free argv before early return since it was allocated
memory. Need to be careful since it may be possible for ss_parse() to have
freed the memory allocated to it if it detects an unbalanced set of quotes
passed to it.

Coverity ID: 21: Resource Leak

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>