bugfix: invalid write in ffi.copy since we missed the len argument, it is caught by the valgrind mode in ec2 testing. (openresty#325)

Author: zhuizhuhaomeng

lib/resty/core/utils.lua

@@ -33,7 +33,7 @@ if subsystem == "http" then
 
  local len = #str
  local buf = get_string_buf(len)
- ffi_copy(buf, str)
+ ffi_copy(buf, str, len)
 
  C.ngx_http_lua_ffi_str_replace_char(buf, len, byte(find),
  byte(replace))