backends: Simplify host_memory_backend_memory_complete()

Return early if bc->alloc is NULL. De-indent the if() ladder.

Note, this avoids a pointless call to error_propagate() with
errp=NULL at the 'out:' label.

Change trivial when reviewed with 'git-diff --ignore-all-space'.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Message-Id: <20231120213301.24349-16-philmd@linaro.org>