Fix some more crashes at session-close

Fix some more crashes at session-close

PortDrop unregisters backend ports, the backend will return a NULL
buffer-pointer, but the I/O object still exists (Metronome, LTC)

AudioBuffer::_data == 0

#0  msvcrt!memset () from C:\Windows\System32\msvcrt.dll
#1  ARDOUR::AudioBuffer::silence (this=0x2c410710, len=256, offset=0) at ../libs/ardour/audio_buffer.cc:88
#2  ARDOUR::AudioPort::cycle_end (this=0x34918730, nframes=256) at ../libs/ardour/audio_port.cc:66
#3  ARDOUR::PortManager::cycle_end (this=this@entry=0x23342770, nframes=nframes@entry=256)
[process callback w/o session]