basesink: handle step end correctly
basesink: handle step end correctly

when we have a new step event with a -1 amount, make sure that we follow the
regular code path so that the stop_end handler is called as usual. This takes
care of flushing the buffer in case of a flushing step and also posts a step end
message.

See https://bugzilla.gnome.org/show_bug.cgi?id=679378

lubosz/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,collects/gstreamer,cablelabs/gstreamer,centricular/gstreamer,krichter722/gstreamer,surround-io/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,collects/gstreamer,lovebug356/gstreamer,collects/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,justinjoy/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,justinjoy/gstreamer,mparis/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,centricular/gstreamer,cablelabs/gstreamer,ensonic/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,mparis/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,lubosz/gstreamer,cfoch/gstreamer,collects/gstreamer,cfoch/gstreamer,justinjoy/gstreamer,jpxiong/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,krichter722/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,krichter722/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,mparis/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,mparis/gstreamer,krieger-od/gstreamer,Lachann/gstreamer,centricular/gstreamer,Lachann/gstreamer,surround-io/gstreamer,Lachann/gstreamer,krieger-od/gstreamer,surround-io/gstreamer,centricular/gstreamer,ensonic/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,lubosz/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,lovebug356/gstreamer,shelsonjava/gstreamer,collects/gstreamer,mparis/gstreamer,lovebug356/gstreamer,cablelabs/gstreamer,surround-io/gstreamer,justinjoy/gstreamer