[network] fix conn_limit underflow error
[network] fix conn_limit underflow error

Connection limit was underflowing on an unsigned number causing panics.
This should replace it without causing underflow.

Closes: #5084
Approved by: phlip9

aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra