Fix security issue

We are using string functions which do the right thing during dup. So
lets use it. Otherwise we were writing an extra \0 sometimes causing
heap overflow

Author: shailesh33

src/dyn_string.c

@@ -75,14 +75,12 @@ string_duplicate(struct string *dst, const struct string *src)
     ASSERT(dst->len == 0 && dst->data == NULL);
     ASSERT(src->len != 0 && src->data != NULL);
 
-    dst->data = dn_strndup(src->data, src->len + 1);
+    dst->data = dn_strndup(src->data, src->len);
     if (dst->data == NULL) {
         return DN_ENOMEM;
     }
 
-    dst->len = src->len;
-    dst->data[dst->len] = '\0';
-
+    dst->len = dn_strlen(dst->data);
     return DN_OK;
 }
 
@@ -92,13 +90,12 @@ string_copy(struct string *dst, const uint8_t *src, uint32_t srclen)
     //ASSERT(dst->len == 0 && dst->data == NULL);
     ASSERT(src != NULL && srclen != 0);
 
-    dst->data = dn_strndup(src, srclen + 1);
+    dst->data = dn_strndup(src, srclen);
     if (dst->data == NULL) {
         return DN_ENOMEM;
     }
 
-    dst->len = srclen;
-    dst->data[dst->len] = '\0';
+    dst->len = dn_strlen(dst->data);
 
     return DN_OK;
 }