prevents an overflowing subtraction when using JLink

[benbrittain]

When using JLink V762c on Linux with a Cortex-M4, I got a

thread 'main' panicked at 'attempt to subtract with overflow', humility-core/src/core.rs:597:56
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: humility failed

when issuing a cargo xtask humility <app> tasks command

This looks like it's caused by a spurious (or perhaps overlapping?) ack.

[2022-04-03T23:22:37Z TRACE humility_core::core] sending $m200022a8,20#ba
[2022-04-03T23:22:37Z TRACE humility_core::core] response: +
[2022-04-03T23:22:37Z TRACE humility_core::core] response: +$382300201000000001000000602300209cff0000a9830100f887010000000041#62

Checking that the responding message is larger or equal to the expected GDB packet fixes the problem.

humility: attached via JLink
system time = 352367
ID TASK                 GEN PRI STATE    
 0 idle                   0   5 RUNNING
 1 jefe                   0   0 recv, notif: bit0 bit1(T+33)
 2 hiffy                  0   3 notif: bit0(T+133)
 3 gpio                   0   2 recv
 4 blinky                 0   3 recv, notif: bit0(T+133)

[benbrittain]

Removed the accidental code due to a making a pull request from master branch & rebased. Confirmed it still works as intended