sss: mark closed fds as -1 in poll array

When a pin's file descriptor is closed after reading, it must be removed
from the poll set. Unlike epoll (which automatically removes closed fds),
poll() will return immediately with POLLNVAL for closed fds, creating an
infinite loop since we only check for POLLIN/POLLPRI.

Setting the fd to -1 causes poll() to ignore it, matching epoll behavior.

Author: sini

src/pins/sss/clevis-decrypt-sss.c

@@ -260,6 +260,8 @@ main(int argc, char *argv[])
 
             fclose(pin->file);
             pin->file = NULL;
+            /* Remove closed fd from poll set (poll ignores negative fds) */
+            pollfds[pi].fd = -1;
 
             waitpid(pin->pid, NULL, 0);
             pin->pid = 0;