Fix MyThread NULL _reent pointer

[LeonLeBreton]

Hi, while playing around with the Rosalina source code, I noticed that using any stdio function that format strings (like sprintf, snprintf, etc.), when we want to formats something, make the console crash (so this is ok: snprintf(buf, sizeof(buf), "OK");, and this make the console crash: snprintf(buf, sizeof(buf), "CRASH %u", 1u);).

After long research, I found out that the problem is that is because of MyThread.c that don't initialize the TLS variables correctly.

libctru/source/internal.h

typedef struct
{
	// Magic value used to check if the struct is initialized
	u32 magic;

	// Pointer to the current thread (if exists)
	Thread thread_ptr;

	// Pointer to this thread's newlib state
	struct _reent* reent;

	// Pointer to this thread's thread-local segment
	void* tls_tp; // !! Keep offset in sync inside __aeabi_read_tp !!

	// FS session override
	u32    fs_magic;
	Handle fs_session;

	// Whether srvGetServiceHandle is non-blocking in case of full service ports.
	bool srv_blocking_policy;
} ThreadVars;

reent, need for newlib to work correctly, is not initialized in MyThread.c, so when we call snprintf with a format string that needs to access the reent structure, it crashes because the pointer is NULL.

So I modified MyThread.c to initialize the reent pointer using (u32)_impure_ptr, and now snprintf works correctly. (I only added that, but the best would be to initialize the whole structure correctly).

If you want to test the crash, here is a simple snippet that you can add to the Rosalina menu.c:

# #include "luma_config.h"
# 
# #include <stdio.h>
+ static void crasher_proof(void)
+ {
+     char buf[16];
+     snprintf(buf, sizeof(buf), "CRASH %u", 42u);
+     (void)buf;
+ }
#
# Menu rosalinaMenu = {
#     "Rosalina menu",
#     {
+        { "[TEST] Crash proof", METHOD, .method = &crasher_proof },
#        { "Take screenshot", METHOD, .method = &RosalinaMenu_TakeScreenshot },

Trigger the entry immediately crash the console without the fix, and with the fix it works correctly.