Reject channels if the total reserves are larger than the funding

Reject channels if the total reserves are larger than the funding

The `full_stack_target` fuzzer managed to find a subtraction
underflow in the new `Channel::get_htlc_maximum` function where we
subtract both sides' reserve values from the channel funding. Such
a channel is obviously completely useless, so we should reject it
during opening instead of integer-underflowing later.

Thanks to Chaincode Labs for providing the fuzzing resources which
found this bug!