rfbsrc: Fix decide_allocation to support NULL pool

We were assuming that NULL pool meant that downstream didn't reply.
Update the pool index 0 instead of adding at the end. Otherwise we ended
up letting basesrc decide, which would pick the blocksize as a size
(4096) instead of the image size.

https://bugzilla.gnome.org/show_bug.cgi?id=795327
1 file changed