Fix a nigh-impossible overflow in cpuworker.c

Fix a nigh-impossible overflow in cpuworker.c

When we compute the estimated microseconds we need to handle our
pending onionskins, we could (in principle) overflow a uint32_t if
we ever had 4 million pending onionskins before we had any data
about how onionskins take.  Nevertheless, let's compute it properly.

Fixes bug 8210; bugfix on 0.2.4.10. Found by coverity; this is CID
980651.