nir: mark unused space in packed_tex_data

nir: mark unused space in packed_tex_data

This change cleans following scary warnings in valgrind output
when disk cache is being written:

   ==6532== Uninitialised byte(s) found during client check request
   ==6532==    at 0x14423FAD: blob_write_bytes (blob.c:152)
   ==6532==    by 0x144240FB: blob_write_uint32 (blob.c:194)
   ==6532==    by 0x144001A5: write_tex (nir_serialize.c:613)

and later (loads of):

   ==6532== Use of uninitialised value of size 8
   ==6532==    at 0x62FCD9E: crc32_z (in /usr/lib64/libz.so.1.2.11)
   ==6532==    by 0x13F65014: util_hash_crc32 (crc32.c:127)
   ==6532==    by 0x13F5DABA: cache_put (disk_cache.c:947)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>