[BUG] RTCB can't be null since g_readytorun is in bss section

### Description / Steps to reproduce the issue

https://github.com/apache/nuttx/blob/master/sched/sched/sched_lock.c#L80-L85
Mentions that `rtcb may be NULL only during early boot-up phases` however RTCB ends up in bss.
```
20002450 g O .bss 00000008 g_readytorun
```
Therefore rtcb is a random value instead of NULL.

Not sure what would be better remove the sched_lock check or initialize g_readytorun as 0.

### On which OS does this issue occur?

[OS: Linux]

### What is the version of your OS?

Ubuntu

### NuttX Version

Master

### Issue Architecture

[Arch: arm]

### Issue Area

[Area: OS Components]

### Verification

- [X] I have verified before submitting the report.