glupload: fix segfault
Without this fix, it is possible that outbuf is not initialized, which will result in segfault when call gst_buffer_replace (&outbuf, NULL). In addition, the patch fixes potential memory leak in restart path.
The segfault can be reproduced by the pipeline below:
GST_GL_PLATFORM=egl
gst-launch-1.0 videotestsrc ! msdkh265enc ! msdkh265dec !
'video/x-raw(memory:DMABuf)' ! glimagesink