file.c: Optimize `rb_file_dirname_n` fixed costs

- `str_null_check` was performed twice, once by `FilePathStringValue`
  and a second time by `StringValueCStr`.
- `StringValueCStr` was checking for the terminator presence, but we
  don't care about that.
- `FilePathStringValue` calls `rb_str_new_frozen` to ensure `fname`
  isn't mutated, but that's costly for such a check. Instead we
  can do it in debug mode only.
- `rb_enc_get` is slow because it accepts arbitrary objects, even immediates,
  so it has to do numerous type checks. Add a much faster `rb_str_enc_get`
  when we know we're dealing with a string.
- `rb_enc_copy` is slow for the same reasons, since we already have the
  encoding, we can use `rb_enc_str_new` instead.

1 files changed, 1 insertions, 1 deletions

diff --git a/string.c b/string.c
index 1e0b9929ef..464eab2146 100644
--- a/string.c
+++ b/string.c
@@ -2880,7 +2880,7 @@ str_null_check(VALUE str, int *w)
     int minlen = 1;
 
     if (RB_UNLIKELY(!rb_str_enc_fastpath(str))) {
-        rb_encoding *enc = rb_enc_get(str);
+        rb_encoding *enc = rb_str_enc_get(str);
         minlen = rb_enc_mbminlen(enc);
 
         if (minlen > 1) {