try to fix a crash OMC is having.

git-svn-id: https://svn.code.sf.net/p/fteqw/code/trunk@4508 fc73d0e0-1445-4013-8a0c-d673dee63da5

engine/gl/gl_backend.c

@@ -1344,6 +1344,9 @@ void GLBE_Init(void)
 					"}\n"
 				"}\n"
 			);
+	//if the shader doesn't work, give up with that.
+	if (!shaderstate.depthonlyshader || !shaderstate.depthonlyshader->prog)
+		shaderstate.depthonlyshader = NULL;
 
 	//make sure the world draws correctly
 	r_worldentity.shaderRGBAf[0] = 1;