lxc-unshare asprintf error checking

same correction in lxc_unshare.c as already done
in lxc_start.c and lxc_restart.c by Nathan in
commit fa9ab20562

Signed-off-by: Michel Normand <michel.mno@free.fr>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>

src/lxc/lxc_unshare.c

@@ -205,7 +205,7 @@ int main(int argc, char *argv[])
 		return -1;
 	}
 
-	if (!asprintf(&pid_name, "%d", pid)) {
+	if (asprintf(&pid_name, "%d", pid) == -1) {
 		ERROR("pid_name: failed to allocate memory");
 		return -1;
 	}