Fixes a bug in the mini_miner_selection fuzz test found by fuzzing:
It was possible for the mini_miner_selection fuzz test to generated
transactions that created fewer new spendable outputs than the two
inputs they each spend. If the fuzz seed did so consistently, eventually
it would cause a `pop_front()` on an empty available_coins.
Fixed by:
- asserting that available_coins is not empty before generating tx
- allowing to build tx with a single coin if only one is available
