FFI: initializing a VLA from a table writes past the end of the allocation

[chernetskyi]

Environment: v2.1 @ c6ffc14, all targets. Reproduced on macOS arm64, with and without the JIT.

local ffi = require("ffi")
local a = ffi.new("int[?]", 2, {1, 2, 3, 4, 5, 6, 7, 8})
print(ffi.sizeof(a), a[2])

Expected: too many initializers for 'int [?]'. The flat-list form already does this: ffi.new("int[?]", 2, 1, 2, 3) raises the error.

Actual: all eight elements are written into the 8-byte allocation, with no error. With AddressSanitizer (-DLUAJIT_USE_SYSMALLOC):

ERROR: AddressSanitizer: heap-buffer-overflow
WRITE of size 4 ...
    #1 lj_cconv_ct_ct lj_cconv.c
    #2 lj_cconv_ct_tv lj_cconv.c:641
    #3 cconv_array_tab lj_cconv.c:484
    #4 lj_cf_ffi_new lib_ffi.c:510
... is located 0 bytes after 32-byte region

The VLA constructor from ffi.typeof("int[?]") behaves the same way.