nir/opt_deref: don't try to cast empty structures

Found while running valgrind :

==3583454== Invalid read of size 4
==3583454==    at 0xF48336: glsl_get_struct_field_offset (nir_types.cpp:84)
==3583454==    by 0xC7CD0D: opt_replace_struct_wrapper_cast (nir_deref.c:1068)
==3583454==    by 0xC7CDD9: opt_deref_cast (nir_deref.c:1087)
==3583454==    by 0xC7DD8E: nir_opt_deref_impl (nir_deref.c:1369)
==3583454==    by 0xC7DF4E: nir_opt_deref (nir_deref.c:1428)
==3583454==    by 0xA63F3C: brw_kernel_from_spirv (brw_kernel.c:325)
==3583454==    by 0xA3BC2C: main (intel_clc.c:481)
==3583454==  Address 0xe4f7e88 is 24 bytes after a block of size 48 in arena "client"

Signed-off-by: Lionel Landwerlin lionel.g.landwerlin@intel.com Cc: mesa-stable