fix: avoid inspecting released shader slots

In some circumstances like loading another texture into an image handle
some shader slots could become NULL and break code that assumed that all
valid handles were densely packed.

Author: zao

engine/render/r_main.cpp

@@ -1131,10 +1131,11 @@ void r_renderer_c::PumpShaders()
 {
 	texMan->ProcessPendingTextureUploads();
 	for (size_t idx = 0; idx < numShader; ++idx)
-		if (auto tex = shaderList[idx]->tex; tex && tex->status != r_tex_c::DONE) {
-			inhibitElision = true;
-			break;
-		}
+		if (auto* sh = shaderList[idx])
+			if (auto tex = sh->tex; tex && tex->status != r_tex_c::DONE) {
+				inhibitElision = true;
+				break;
+			}
 }
 
 void r_renderer_c::BeginFrame()