st/mesa: move dereference after null check
st/mesa: move dereference after null check

We should not dereference shader before we have done the
null check.

Reviewed-by: Erik Faye-Lund <2a8ffc07e8f57d80a2934ccc3dd1949354d82b2d@gmail.com>
Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler