st/mesa: fix member access within null pointer check_program_state
I was able to hit this error when playing openjk using crocus.
../src/mesa/state_tracker/st_atom.c:94:8: runtime error: member access within null pointer of type 'struct st_program'
../src/mesa/state_tracker/st_atom.c:101:8: runtime error: member access within null pointer of type 'struct st_program'
../src/mesa/state_tracker/st_atom.c:108:8: runtime error: member access within null pointer of type 'struct st_program'
../src/mesa/state_tracker/st_atom.c:115:8: runtime error: member access within null pointer of type 'struct st_program'
Idea for fix has been taken from code a few lines above. Line 87.