read_config: fix leak on error
read_config: fix leak on error

Found through static analysis.

1ace/sway,ascent12/sway,ascent12/sway,taiyu-len/sway,ascent12/sway,SirCmpwn/sway,taiyu-len/sway,1ace/sway,1ace/sway,taiyu-len/sway