Fix: check the result iterator init in print_report_port_xml

[mattmundell]

What

In print_report_port_xml, check init_result_get_iterator and, when it
fails, return without using the iterator, leaving a valid empty iterator for
the caller to clean up.

Why

print_report_port_xml ignored init_result_get_iterator's return and then
ran next on the iterator. When the init failed before touching the iterator,
then next and the caller's cleanup_iterator used uninitialised stack memory.

Testing

Before the change, an authenticated

<get_report_ports report_id="..." filt_id="F" details="1"/>

for a report with ports, whose filter F was deleted between the request's
filter validation and the port-iterator init, made gvmd dereference
the uninitialised iterator and abort:

==383==ERROR: AddressSanitizer: SEGV on unknown address ...
    #0 sql_finalize /home/matt/src/gvmd/src/sql_pg.c:766
    #1 cleanup_iterator /home/matt/src/gvmd/src/sql.c:1234
    #2 print_report_port_xml_summary_or_details
        /home/matt/src/gvmd/src/manage_sql_report_ports.c:470
    #3 manage_send_report_ports
        /home/matt/src/gvmd/src/manage_report_ports.c:131
    #4 get_report_ports_run /home/matt/src/gvmd/src/gmp_report_ports.c:155
SUMMARY: AddressSanitizer: SEGV /home/matt/src/gvmd/src/sql_pg.c:766
    in sql_finalize

After the change the same request returns with no ASan report.