s3:libsmb: Check if we have a valid file descriptor

s3:libsmb: Check if we have a valid file descriptor

"Error: REVERSE_NEGATIVE (CWE-191):
samba-4.20.0rc2/source3/libsmb/pylibsmb.c:215: negative_sink_in_call: Passing ""t->shutdown_pipe[1]"" to a parameter that cannot be negative.
samba-4.20.0rc2/source3/libsmb/pylibsmb.c:230: check_after_sink: You might be using variable ""t->shutdown_pipe[1]"" before verifying that it is >= 0.
  228|   		t->shutdown_pipe[0] = -1;
  229|   	}
  230|-> 	if (t->shutdown_pipe[1] != -1) {
  231|   		close(t->shutdown_pipe[1]);
  232|   		t->shutdown_pipe[1] = -1;"

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Martin Schwenke <mschwenke@ddn.com>