This was crashing using the clone tool without a clone image assigned.

Caused by rB9111ea78acf4: Localize image mutex lock into runtime field of Image datablock.
Since above commit, BKE_image_acquire_ibuf was using ima->runtime
without checking for NULL first.
Since callers are not required to check for this, just return early
here.

note: there is still a memory leak using the clone tool without a clone
image assigned (but this was also the case before said commit and needs
to be investigated separately).