fix: reject oversized state in deserialize

[dekobon]

This PR seeks to fix #368 where in src/scanner.c:135-143, we have the following function:

void tree_sitter_cpp_external_scanner_deserialize(void *payload, const char *buffer, unsigned length) {
    assert(length % sizeof(wchar_t) == 0 && "Can't decode serialized delimiter!");

    Scanner *scanner = (Scanner *)payload;
    scanner->delimiter_length = length / sizeof(wchar_t);
    if (length > 0) {
        memcpy(&scanner->delimiter[0], buffer, length);
    }
}

Here the greater than zero check is insufficiently guarding memcpy.

The variable Scanner is { uint8_t delimiter_length; wchar_t delimiter[16] } (68 bytes total on Linux/macOS, of which 64 are the delimiter; 32 where wchar_t is 2 bytes).

Elsewhere in the file we check to ensure that delimiter_length <= MAX_DELIMITER_LENGTH, but only in this function are we able to break that contract.

I've taken this through ASan and have a PoC of breaking through this guard, but in interest of safety, I think this is all the information needed to see why we need a better guard here.

AI disclosure: used for debugging and running ASan, formatting, and verification - poorly written prose is 100% organic human

Thank you for the great work with tree-sitter!

Downstream issue prompting PR: dekobon/big-code-analysis#1058
Fixes #368