* fns.c (Flength): Don't overflow int when computing a list length.

* fns.c (Flength): Don't overflow int when computing a list length.

Use EMACS_INT, not int, to avoid unwanted truncation on 64-bit hosts.
Check for QUIT every 1024 entries rather than every other entry;
that's faster and is responsive enough.  Report an error instead of
overflowing an integer.