gl_array_nx_create: list->count set too late

[Alexander Sosedkin]

Hello, apologies for a terse report, hope it still comes useful.

Since 528000 elements in struct gl_list_impl are __counted_by(count).
gl_array_nx_create was probably overlooked
and setting the count wasn't hoisted to before the memcpy.
Clang 21 is either too smart or too dumb to consider it a problem,
but gcc 16.2.0 + _FORTIFY_SOURCE=3, the current nixpkgs defaults,
have four list tests failing because of that.
Moving list->count = count; should be enough to fix it.