Fix crash caused by small buffer

Plasmashell will crash on wayland, when change vlc media by mpris.

ref: libplasma!1068

backtrace:

crash code:

params is created in PipeWireSourceStream::onRenegotiate

add some logs to prove this:

But I don't think changing 4096 to 8192 is a good way to solve the problem,

Maybe 8192 will be not large enough in someday.