rgw/rgw_admin.cc: rework read_input() to fix resource leak

Rework function to prevent leakage of file descriptors. Don't do
anything if no file name is defined or the file couldn't get opened.

CID 1019581 (#1 of 1): Resource leak (RESOURCE_LEAK)
  leaked_handle: Handle variable "fd" going out of scope leaks the handle.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>