The variant format string `^aay` is said to be equivalent to
`g_variant_new_bytestring_array`. Given that no length parameter is
provided, glib assumed a null-terminated array, causing a crash as glib
exceed the read boundaries to copy arbitrary memory.
This commit replaces the array construction code to use its arena
equivalents instead of glib, and make sure that the resulting array is
null-terminated.
Fixes#3616.
c3ef4d2908