st/mesa: fix a segfault when prog->sh.data is NULL
st/mesa: fix a segfault when prog->sh.data is NULL

Broken by:
   st/mesa: get Version from gl_program rather than gl_shader_program

Reviewed-by: Samuel Pitoiset <506e97eed81d9e59d22f4c73fb51123bb62b4a30@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler