tests/examples/playlist.c: better sanity checking of arguments
tests/examples/playlist.c: better sanity checking of arguments

cfoch/gst-editing-services,luisbg/gst-editing-services,MathieuDuponchelle/PitiviGes,freesteph/gst-editing-services,MathieuDuponchelle/PitiviGes,volodymyrrudyi/gst-editing-services,MathieuDuponchelle/PitiviGes,freesteph/gst-editing-services,cfoch/ges,pitivi/gst-editing-services,pitivi/gst-editing-services,ahmedammar/platform_external_gst_editing_services,cfoch/ges,luisbg/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,pitivi/gst-editing-services,volodymyrrudyi/gst-editing-services,pitivi/gst-editing-services,MathieuDuponchelle/PitiviGes,cfoch/gst-editing-services,cfoch/ges,ahmedammar/platform_external_gst_editing_services,cfoch/gst-editing-services,volodymyrrudyi/gst-editing-services,freesteph/gst-editing-services