Prevent double-free in attemptDirectScanout

[Kam1k4dze]

Fixes #10940

This is what came to my mind after looking at the asan #10940 (comment)
Sorry, if this is not right way to fix that.

[gulafaran]

just to keep it somewhat like the other lambda captures does i would do

PBUFFER->onBackendRelease([weakBuffer = WP<IHLBuffer>(PBUFFER)]() {
    if (!weakBuffer)
         return;
         
    weakBuffer->unlock();
});

but should work the same practically. 👍