Check if streams array exists in handle_draw_jpeg_onload

• It seems SpiceDisplayConn does not always have the array this.o.sc.streams set.
• It also seems (stream?) images can be loaded before streams is set.
• Without streams, or the specific stream matching this.o.id, this.o.sc.streams[this.o.id].frames_loading cannot be accessed.
• The check for the specific stream woth this.o.id is already in place, this patch adds a check for this.o.sc.streams in handle_draw_jpeg_onload.
• There might be a better place to ensure that this.o.sc.streams is initialized; this quick fix saved my bacon today though.

Might be related to, and perhaps fix:

• https://bugs.freedesktop.org/show_bug.cgi?id=94776
• https://bugzilla.redhat.com/show_bug.cgi?id=1323144

Signed-off-by: Joel Purra mig@joelpurra.se