Fix memory error on str_buf_cat

Modify from nofree to embed string

src/string.c

@@ -695,14 +695,21 @@ mrb_str_modify(mrb_state *mrb, struct RString *s)
   }
   if (RSTR_NOFREE_P(s)) {
     char *p = s->as.heap.ptr;
+    mrb_int len = s->as.heap.len;
 
-    s->as.heap.ptr = (char *)mrb_malloc(mrb, (size_t)s->as.heap.len+1);
+    RSTR_UNSET_NOFREE_FLAG(s);
+    if (len < RSTRING_EMBED_LEN_MAX) {
+      RSTR_SET_EMBED_FLAG(s);
+      RSTR_SET_EMBED_LEN(s, len);
+    }
+    else {
+      s->as.heap.ptr = (char *)mrb_malloc(mrb, (size_t)len+1);
+      s->as.heap.aux.capa = len;
+    }
     if (p) {
-      memcpy(RSTR_PTR(s), p, s->as.heap.len);
+      memcpy(RSTR_PTR(s), p, len);
     }
-    RSTR_PTR(s)[s->as.heap.len] = '\0';
-    s->as.heap.aux.capa = s->as.heap.len;
-    RSTR_UNSET_NOFREE_FLAG(s);
+    RSTR_PTR(s)[len] = '\0';
     return;
   }
 }