tracing/osnoise: Fix null-ptr-deref in bitmap_parselist()

In the Linux kernel, the following vulnerability has been resolved:

tracing/osnoise: Fix null-ptr-deref in bitmap_parselist()

A crash was observed with the following output:

BUG: kernel NULL pointer dereference, address: 0000000000000010 Oops: Oops: 0000 [#1] SMP NOPTI CPU: 2 UID: 0 PID: 92 Comm: osnoise_cpus Not tainted 6.17.0-rc4-00201-gd69eb204c255 #138 PREEMPT(voluntary) RIP: 0010:bitmap_parselist+0x53/0x3e0 Call Trace:

osnoise_cpus_write+0x7a/0x190 vfs_write+0xf8/0x410 ? do_sys_openat2+0x88/0xd0 ksys_write+0x60/0xd0 do_syscall_64+0xa4/0x260 entry_SYSCALL_64_after_hwframe+0x77/0x7f

This issue can be reproduced by below code:

fd=open("/sys/kernel/debug/tracing/osnoise/cpus", O_WRONLY); write(fd, "0-2", 0);

When user pass 'count=0' to osnoise_cpus_write(), kmalloc() will return ZERO_SIZE_PTR (16) and cpulist_parse() treat it as a normal value, which trigger the null pointer dereference. Add check for the parameter 'count'.