str, bench: three checks that guard nothing

sinit sets s->n = 0 as its first statement and no failure path restores
it, so stail's sclear after a failed sinit could never change anything;
say so in sinit's comment instead, where the contract belongs.  bench
compared a uintmax_t against UINT64_MAX and a size_t against UINT64_MAX,
both constant on any host this builds for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

str.c

@@ -1,7 +1,8 @@
 #include "dat.h"
 #include "fn.h"
 
-/* Fills s from n bytes of UTF-8; whole, valid, and at most Maxrunes. */
+/* Fills s from n bytes of UTF-8; whole, valid, and at most Maxrunes.
+ * Anything else leaves s empty. */
 int
 sinit(Str *s, char *src, int n)
 {
@@ -35,8 +36,7 @@ stail(Str *s, char *src, int n)
 	nr = utfnlen(src, n);
 	for(p = src; nr > Maxrunes; nr--)
 		p += chartorune(&r, p);
-	if(!sinit(s, p, n - (p - src)))
-		sclear(s);
+	sinit(s, p, n - (p - src));
 }
 
 void