wayland-util: avoid memcpy(NULL) in wl_array_copy()

The problem was found running Weston, with both Weston and Wayland built
with ASan:

../../git/wayland/src/wayland-util.c:150:2: runtime error: null pointer passed as argument 1, which is declared to never be null
../../git/wayland/src/wayland-util.c:150:2: runtime error: null pointer passed as argument 2, which is declared to never be null

This turns out to be caused by copying an empty array into an empty
array.

That seems to be completely valid thing to do, and wl_array_init()
initializes the pointers to NULL and size to zero. Copying initialized
arrays must always be valid.

The error are caused by calling memcpy() with NULL pointers. It doesn't
explode, because also the size is zero.

Fix the problem by calling memcpy() only if size is not zero. This
should keep things like copying an empty array into a non-empty array
work.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

src/wayland-util.c

@@ -147,7 +147,9 @@ wl_array_copy(struct wl_array *array, struct wl_array *source)
 		array->size = source->size;
 	}
 
-	memcpy(array->data, source->data, source->size);
+	if (source->size > 0)
+		memcpy(array->data, source->data, source->size);
+
 	return 0;
 }