check: Check for pipe return value in check_unaligned_vmsplice

| cr-check.c: In function ‘check_unaligned_vmsplice’:
| cr-check.c:372:2: error: ignoring return value of ‘pipe’, declared with attribute warn_unused_result [-Werror=unused-result]

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>