Fix sigaltstack error path leaking sighand lock

[SongXiaoXi]

sys_sigaltstack() returns _ENOMEM while still holding sighand->lock when the new alt stack is smaller than MINSIGSTKSZ_, causing a permanent lock leak/deadlock on later signal operations.

A simple reproducer:

#define _GNU_SOURCE
#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>
int main(void) {
    setvbuf(stdout, NULL, _IONBF, 0);
    stack_t ss = {0};
    ss.ss_sp = malloc(1);
    ss.ss_size = 1; // intentionally smaller than MINSIGSTKSZ
    ss.ss_flags = 0;
    printf("[1] calling sigaltstack(tiny stack)...\n");
    errno = 0;
    long r = syscall(SYS_sigaltstack, &ss, NULL);
    printf("[1] ret=%ld errno=%d (%s)\n", r, errno, strerror(errno));
    return 0;
}

When this reproducer is compiled and run inside the iSH app, the whole app becomes stuck after the first failing sigaltstack() call.