fix: harden small core containers

tests/str_test.c

@@ -82,3 +82,21 @@ str_utf8_capacity(struct ct *t)
 	CT_EQ_STR(t, "😀", buf);
 	CT_EQ_INT(t, 'Z', buf[5]);
 }
+
+void
+str_invalid_and_full_appends(struct ct *t)
+{
+	Str s;
+	int i;
+
+	sinit(&s, nil, 1);
+	CT_EQ_INT(t, 0, s.n);
+	sinit(&s, "x", -1);
+	CT_EQ_INT(t, 0, s.n);
+	for(i = 0; i < Maxrunes; i++)
+		s.r[i] = 'a';
+	s.n = Maxrunes;
+	sputr(&s, 'z');
+	CT_EQ_INT(t, Maxrunes, s.n);
+	CT_EQ_INT(t, 'a', s.r[Maxrunes-1]);
+}