fix: harden small core containers

str.c

@@ -7,6 +7,8 @@ sinit(Str *s, char *src, int n)
 	int len;
 
 	s->n = 0;
+	if(n < 0 || (n > 0 && src == nil))
+		return;
 	while(n > 0 && s->n < Maxrunes){
 		if(!fullrune(src, n))
 			break;
@@ -28,8 +30,9 @@ sclear(Str *s)
 void
 sputr(Str *s, Rune r)
 {
+	/* Str is a capped value; appends at capacity leave it unchanged. */
 	if(s->n >= Maxrunes)
-		die("sputr overflow");
+		return;
 	s->r[s->n++] = r;
 }