SL-20730 Patch another potential source of NaNs

indra/newview/app_settings/shaders/class1/deferred/emissiveF.glsl

@@ -34,6 +34,6 @@ void main()
 {
     // NOTE: when this shader is used, only alpha is being written to
 	float a = diffuseLookup(vary_texcoord0.xy).a*vertex_color.a;
-	frag_color = vec4(0, 0, 0, a);
+	frag_color = max(vec4(0, 0, 0, a), vec4(0));
 }