obexd: Fix not checking for valid fd on NewConnection

The fd needs to be checked as it may not be valid which cause the
following warnings:

==8162== Warning: invalid file descriptor 1031 in syscall fcntl(DUPFD_CLOEXEC)()

(obexd:8162): GLib-WARNING **: giounix.c:412Error while getting flags for FD: Bad file descriptor (9)
1 file changed