wined3d: Make sure the p8 shader is set to 0 after destroying the main render target.

This fixes a render target recreation problem in Age of Empires II.

dlls/wined3d/device.c

diff --git a/dlls/wined3d/device.c b/dlls/wined3d/device.c
index e9cd3de..ef6a0ea 100644
--- a/dlls/wined3d/device.c
+++ b/dlls/wined3d/device.c
@@ -2163,6 +2163,7 @@
     /* Delete the palette conversion shader if it is around */
     if(This->paletteConversionShader) {
         GL_EXTCALL(glDeleteProgramsARB(1, &This->paletteConversionShader));
+        This->paletteConversionShader = 0;
     }
 
     /* Delete the pbuffer context if there is any */