gstreamer: allocator: gst_libcamera_allocator_new(): Recognize errors

`FrameBufferAllocator::allocate()` might return a negative error code,
but currently this is handled the same way as success. So instead
of continuing, abort the construction of the allocator object.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>

src/gstreamer/gstlibcameraallocator.cpp

@@ -214,7 +214,7 @@ gst_libcamera_allocator_new(std::shared_ptr<Camera> camera,
 		Stream *stream = streamCfg.stream();
 
 		ret = self->fb_allocator->allocate(stream);
-		if (ret == 0)
+		if (ret <= 0)
 			return nullptr;
 
 		GQueue *pool = g_queue_new();