Fix panic in FundManager

[dirkmc]

I believe the issue can occur in this scenario:

1. Deal A adds 5 to addr1:                     reserved  0 ->  5    available  0 ->  5
2. Deal B adds 7 to addr1:                     reserved  5 -> 12    available  5 -> 12
3. Deal B completes, reducing addr1 by 7:      reserved 12 -> 12    available 12 ->  5
4. Deal A releases 5 from addr1:               reserved 12 ->  7    available  5 ->  5

In step 4: reserved (7) - available (5) > 0 and len(toAdd) == 0

[jsign]

I can confirm that the pipeline I ran to discover this stopped panicking, so looks good.

[hannahhoward]

I am a bit anxious about the sequence of things that gets us to this condition -- how would we end up with a reserved > available but no toAdd messages? Worth possible further investigation. That said, no panic is better than panic.

[arajasek]

Makes sense

[dirkmc]

Updated the description with a scenario in which this panic can occur