kms: fix crash if bo allocation failed when copying to dumb buffer

kms: fix crash if bo allocation failed when copying to dumb buffer

If bo allocation failed we destroy the buffer and return GST_FLOW_ERROR,
but the @buffer pointer was still pointing to the address of the
destroyed buffer. gst_kms_sink_copy_to_dumb_buffer() was then trying to
unref it when bailing out causing a crash.

Leave @buffer untouched if allocation failed to fix the crash.
Also remove the check on *buffer being not NULL as gst_buffer_new()
will abort if it failed.

https://bugzilla.gnome.org/show_bug.cgi?id=787442