fix ngx_utf8_cpystrn(): it did not fully copy utf-8 string

src/core/ngx_string.c

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 613ce93..8299134 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -1239,10 +1239,8 @@
             break;
         }
 
-        len--;
-
         while (src < next) {
-            *++dst = *++src;
+            *dst++ = *src++;
             len--;
         }
     }