unicode:characters_to_list/binary return an uninitialized term for a non-chardata argument #2446

Found while reviewing #2357; pre-existing (same behaviour on release-0.7 at 18b3587). BEAM raises badarg, AtomVM returns an {error, _, Rest} tuple whose Rest is an uninitialized stack value:

unicode:characters_to_list(id(foo))         %% BEAM badarg   AtomVM {error,[],<garbage>}
unicode:characters_to_binary(id({1, 2}))    %% BEAM badarg   AtomVM {error,<<>>,<garbage>}
unicode:characters_to_list(id(<<1:1>>))     %% same, once #2357 makes non-byte-aligned bitstrings possible

erlang:display/1 prints the third element as Unknown term type: 129947828638076, and valgrind reports "Conditional jump or move depends on uninitialised value(s)" in term_is_tuple and memory_shallow_copy_term as soon as the tuple is printed or copied by the GC, so a process keeping the result can crash the VM. A non-chardata element inside a list ([foo]) is correctly rejected with badarg; only the top-level argument is affected.

Cause: interop_chardata_fold (src/libAtomVM/interop.c:252-258) returns InteropBadArg for a top-level term that is neither a binary nor a list before rest_fun is ever called, so *rest is never written. interop_chardata_to_bytes maps that result to UnicodeError (the badarg flag is only set by rest_fun), and nif_unicode_characters_to_list (src/libAtomVM/nifs.c:7278-7279) and nif_unicode_characters_to_binary (:7339-7340) build the result tuple with the uninitialized rest.

Fix: validate argv[0] with term_is_list(t) || term_is_binary(t) in the two NIFs and raise badarg otherwise, or have interop_chardata_fold report the top-level term through rest_fun so that interop_chardata_to_bytes_size and interop_chardata_to_bytes return UnicodeBadArg