Doing a `mkdir` on devices not in the devoptab results in a crash

[mrpapersonic]

Bug Report

What's the issue you encountered?

I was attempting to add support for external USB drives within Schism Tracker and came upon a crash; setting a breakpoint in Cemu shows that the crash was in mkdir after a call to FindDevice. This bug can be seen more explicitly in the source code:

newlib/libgloss/libsysbase/mkdir.c

Lines 8 to 11 in 4a5c7c7

	int dev = FindDevice(path);
	ret = -1;
	if (devoptab_list[dev]->mkdir_r) {

There is no check for whether FindDevice actually returns a valid device.

How can the issue be reproduced?

int main(void)
{
        mkdir("fake:/test");
}

Environment?

Latest versions of devkitPPC and wut directly from devkitpro pacman repos. I'm using Arch Linux, but this can also be replicated on recent GitHub Actions builds using devkitPPC docker containers.

Additional context?

N/A