usb: gadget: f_fs: Guard epfile->error

epfile->error is a QC extension. It needs to be guarded
from being null to work safely with the no_disconnect patch.

Bug: 37423404
Change-Id: I3142a03ef3296b928aa36c54a5397afbe30798b7
Signed-off-by: Jerry Zhang <zhangjerry@google.com>