Use rb_len_t for the line length in rb_io_getline_fast

It was int, which overflows on a line longer than INT_MAX bytes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

io.c

@@ -4260,7 +4260,7 @@ static VALUE
 rb_io_getline_fast(rb_io_t *fptr, rb_encoding *enc, int chomp)
 {
     VALUE str = Qnil;
-    int len = 0;
+    rb_len_t len = 0;
     rb_len_t pos = 0;
     int cr = 0;