SSH2: close channel and break after exit-signal

Prior to https://github.com/phpseclib/phpseclib/commit/efdec7b118b1f8c24b9882ccc4a7fce5cb8d945f exit-signal's would re-use exit-status's channel closing code. after that commit, however, exit-status specific code was added, which meant that falling through, without the break, broke things