rb_file_join_fastpath: optimize searching for separators

`chompdirsep` searches from the start of the string each time, which
perhaps is necessary for certain encodings (not even sure?) but for
the common encodings it's very wasteful. Instead we can start from the
back of the string and only compare one or two characters in most cases.

Also replace `StringValueCStr` for the simpler `rb_str_null_check`
as we only care about whether the string contains `NULL` bytes, we
don't care whether it is NULL terminated or not.

We also only check the final string for NULLs.

```
compare-ruby: ruby 4.1.0dev (2026-01-17T14:40:03Z master 00a3b71eaf) +PRISM [arm64-darwin25]
built-ruby: ruby 4.1.0dev (2026-01-18T12:55:15Z spedup-file-join 5948e92e03) +PRISM [arm64-darwin25]
warming up....

|              |compare-ruby|built-ruby|
|:-------------|-----------:|---------:|
|two_strings   |      2.477M|   19.317M|
|              |           -|     7.80x|
|many_strings  |    547.577k|   10.298M|
|              |           -|    18.81x|
|array         |    515.280k|  523.291k|
|              |           -|     1.02x|
|mixed         |    621.840k|  635.422k|
|              |           -|     1.02x|
```

1 files changed, 28 insertions, 0 deletions

diff --git a/string.c b/string.c
index cfadabd379..1e0b9929ef 100644
--- a/string.c
+++ b/string.c
@@ -2902,6 +2902,34 @@ str_null_check(VALUE str, int *w)
     return s;
 }
 
+const char *
+rb_str_null_check(VALUE str)
+{
+    RUBY_ASSERT(RB_TYPE_P(str, T_STRING));
+
+    char *s;
+    long len;
+    RSTRING_GETMEM(str, s, len);
+
+    if (RB_LIKELY(rb_str_enc_fastpath(str))) {
+        if (!s || memchr(s, 0, len)) {
+            rb_raise(rb_eArgError, "string contains null byte");
+        }
+    }
+    else {
+        int w;
+        const char *s = str_null_check(str, &w);
+        if (!s) {
+            if (w) {
+                rb_raise(rb_eArgError, "string contains null char");
+            }
+            rb_raise(rb_eArgError, "string contains null byte");
+        }
+    }
+
+    return s;
+}
+
 char *
 rb_str_to_cstr(VALUE str)
 {