if mpCurrentProgram and pProgram are NULL this crashes

See
http://crashreport.libreoffice.org/stats/signature/OpenGLProgram::Reuse%28%29

Change-Id: I2d1c9d9faff05d0d57a2c217a9ee594b1c22b61d
Reviewed-on: https://gerrit.libreoffice.org/27043Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

vcl/source/opengl/OpenGLContext.cxx

@@ -816,7 +816,7 @@ OpenGLProgram* OpenGLContext::UseProgram( const OUString& rVertexShader, const O
 
     OpenGLProgram* pProgram = GetProgram( rVertexShader, rFragmentShader, preamble );
 
-    if (pProgram == mpCurrentProgram)
+    if (pProgram && pProgram == mpCurrentProgram)
     {
         VCL_GL_INFO("Context::UseProgram: Reusing existing program " << pProgram->Id());
         pProgram->Reuse();