v4l2videodec: Protect double calls to set_format()
v4l2videodec: Protect double calls to set_format()

In some cases, set_format() may get called twice before the output
format is set. Running an allocation query in this case is both not
needed and will cause assertion due tot he NULL caps.

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