Fix NULL pointer dereference in RE_Shutdown when renderer interface is not initialized

[runlevel5]

Summary

• Add NULL guards for ri.GLimp_Shutdown and ri.VKimp_Shutdown calls in RE_Shutdown() across all three renderers (opengl, opengl2, vulkan)

Problem

When the client binary is built without USE_OPENGL_API (e.g. RENDERER_DEFAULT=vulkan with USE_RENDERER_DLOPEN=0) but later loads the OpenGL renderer .so at runtime, ri.GLimp_Init is NULL. The renderer correctly detects this at InitOpenGL() and calls ri.Error(ERR_FATAL, "OpenGL interface is not initialized"), but the resulting shutdown path in RE_Shutdown() calls ri.GLimp_Shutdown() without checking for NULL, causing a SIGSEGV at address 0x0.

The same issue applies to ri.VKimp_Shutdown in the vulkan renderer's #ifdef USE_VULKAN path and to ri.GLimp_Shutdown in its #else fallback path.

Crash backtrace

#0  0x0000000000000000 in ?? ()
#1  RE_Shutdown (code=REF_UNLOAD_DLL) at code/renderer/tr_init.c:1935
#2  CL_ShutdownRef (code=REF_UNLOAD_DLL) at code/client/cl_main.c:3174
#3  CL_Shutdown (...) at code/client/cl_main.c:4083
#4  Com_Error (code=ERR_FATAL, fmt="OpenGL interface is not initialized") at code/qcommon/common.c:397
#5  InitOpenGL () at code/renderer/tr_init.c:588

GDB confirms the function pointers are NULL:

GLimp_Init = 0x0
GLimp_Shutdown = 0x0
GLimp_EndFrame = 0x0
GL_GetProcAddress = 0x0
VKimp_Init = 0x10173890 <VKimp_Init>     // populated
VKimp_Shutdown = 0x10173c3c <VKimp_Shutdown>  // populated

Fix

Simple NULL guards before the function pointer calls in RE_Shutdown():

• code/renderer/tr_init.c — guard ri.GLimp_Shutdown
• code/renderer2/tr_init.c — guard ri.GLimp_Shutdown
• code/renderervk/tr_init.c — guard both ri.VKimp_Shutdown and ri.GLimp_Shutdown

Testing

Tested on ppc64le (Power9, AMD RX 6600 XT, Fedora 43):

• Built client with USE_OPENGL_API=0, loaded OpenGL renderer: previously crashed with SIGSEGV, now exits cleanly with Sys_Error: OpenGL interface is not initialized
• Both OpenGL and Vulkan renderers still work correctly with a normal build (regression test)