nvc0/ir: prevent out of bounds when no images are bound

nvc0/ir: prevent out of bounds when no images are bound

Checking if the image address is not 0 should be enough to prevent
read faults. To improve robustness, make sure that the destination
value of atomic operations is correctly initialized in case the
instruction is not performed.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>