Fix invalid texture being invalid and causing crash with Vulkan

When starting with `dbg_stress 1` the invalid texture was never actually getting loaded, which was causing the client to crash when using the Vulkan backend and starting with `dbg_stress 1`.

Closes ddnet#6504.

src/engine/client/graphics_threaded.cpp

@@ -423,7 +423,7 @@ IGraphics::CTextureHandle CGraphics_Threaded::LoadTextureRaw(int Width, int Heig
 {
 	// don't waste memory on texture if we are stress testing
 #ifdef CONF_DEBUG
-	if(g_Config.m_DbgStress)
+	if(g_Config.m_DbgStress && m_InvalidTexture.IsValid())
 		return m_InvalidTexture;
 #endif
 
@@ -2903,6 +2903,7 @@ int CGraphics_Threaded::Init()
 		0x00, 0x00, 0xff, 0xff, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff, 0x00, 0xff, 0xff, 0xff, 0x00, 0xff,
 		0x00, 0x00, 0xff, 0xff, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff, 0x00, 0xff, 0xff, 0xff, 0x00, 0xff};
 
+	m_InvalidTexture.Invalidate();
 	m_InvalidTexture = LoadTextureRaw(4, 4, CImageInfo::FORMAT_RGBA, s_aNullTextureData, CImageInfo::FORMAT_RGBA, 0);
 
 	ColorRGBA GPUInfoPrintColor{0.6f, 0.5f, 1.0f, 1.0f};