Buffer: fix type confusion in concat() with element getters

Buffer: fix type confusion in concat() with element getters

njs_buffer_concat() validated each list element as a typed array in the
length pass, then re-read the same elements with njs_value_property_i64()
in the copy pass and cast them with njs_typed_array() without rechecking.
For a non-fast array with an accessor element, a getter returning a typed
array during validation and a non-typed-array (or a detached buffer)
during the copy yielded a wild pointer and an out-of-bounds read.

Revalidate the type and detached state in the copy pass, matching the
length pass and the QuickJS implementation.