load_camera: initialize with calloc to avoid uninitialized reads

in particular: if a sensor can't be found, then camera->sensor_path is
unassigned. this would cause a segfault later, in libmegapixels_open.

src/parse.c

@@ -191,10 +191,7 @@ load_camera(libmegapixels_devconfig *config, config_t *cfg, const char *name)
 		return -1;
 	}
 	libmegapixels_camera *camera;
-	camera = malloc(sizeof(libmegapixels_camera));
+	camera = calloc(1, sizeof(libmegapixels_camera));
-	camera->sensor_fd = 0;
-	camera->media_fd = 0;
-	camera->video_fd = 0;
 
 	int res = find_media_node(camera, bridge_driver, sensor_driver);
 	if (res < 0) {