Fixed Buffer allocation length checks

Fixed Buffer allocation length checks

On 32-bit platforms, where size_t is 32 bits, callers passing
int64_t lengths >= 2^32 to njs_buffer_alloc() had the value
silently truncated before reaching the UINT32_MAX check in
njs_array_buffer_alloc(), so Buffer.from({length: 0x100000000})
returned a zero-sized buffer instead of raising a RangeError.

Widened the size parameter to uint64_t so the value reaches
njs_typed_array_alloc() intact and the "invalid index" range
error fires consistently on 32-bit and 64-bit builds.

Buffer.concat() is protected by the same change: it likewise
reads an int64_t length from JS and forwards it to
njs_buffer_alloc() without an upper bound check of its own.