tests: avoid close of bad file handle in commandtest

Closed file handles need to be initialized to -1, not 0. This caused a
inappropriate double close of stdin, which is not desirable, although
it had no ill effects.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/commandtest.c

@@ -1091,8 +1091,8 @@ static int test27(const void *unused G_GNUC_UNUSED)
         printf("Could not set send buffers\n");
         goto cleanup;
     }
-    pipe1[1] = 0;
-    pipe2[1] = 0;
+    pipe1[1] = -1;
+    pipe2[1] = -1;
     buffer1 = NULL;
     buffer2 = NULL;