souphttpsrc: fix buffer leak when flushing
souphttpsrc: fix buffer leak when flushing

When early returning in gst_soup_http_src_read_buffer() because the
element is FLUSHING, we need to unmap and unref the buffer which was just created.

https://bugzilla.gnome.org/show_bug.cgi?id=766718

GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good