Handle case when pid file is never created

[cjcolvar]

If processing fails during mediainfo file inspection a pid file is never created. When get_pid returns nil it gets converted to 0 by the call to to_i in running? which will always be true. So the encode will never appear as completed.

active_encode/lib/active_encode/engine_adapters/ffmpeg_adapter.rb

Lines 244 to 249 in 3cac5a2

	def running?(pid)
	Process.getpgid pid.to_i
	true
	rescue Errno::ESRCH
	false
	end

I found this with an encode in production and worked around the issue by creating a pid file with -1 as its contents.