blob: Don't valgrind assert for defined memory if we aren't writing.

blob: Don't valgrind assert for defined memory if we aren't writing.

The VK pipeline cache passes a NULL bytes with a nonzero size to a
NULL-data blob to set up the size of the blob.  In this case, we don't
actually execute the memcpy, so the non-existent "bytes" doesn't need to
have defined contents.  Avoids a valgrind warning:

==972858== Unaddressable byte(s) found during client check request
==972858==    at 0x147F4166: blob_write_bytes (blob.c:165)
==972858==    by 0x147F4166: blob_write_bytes (blob.c:158)
==972858==    by 0x14695FFF: vk_pipeline_cache_object_serialize (vk_pipeline_cache.c:240)
[...]
==972858==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

Fixes: 591da9877900 ("vulkan: Add a common VkPipelineCache implementation")
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22617>