Break out of REPL loop if binding_stack.empty?
Break out of REPL loop if binding_stack.empty?

Occasionally pry would go into a broken state where the binding_stack would be empty.
This commit breaks out of the REPL rather than leaving the user in a broken state.

jasnow/pry,gfvcastro/pry,josue804/debunker,geoffreylitt/pry,jasonm23/pry,tdg5/pry,ooohiroyukiooo/pry,geoffreylitt/pry,bronzdoc/pry,scalixte-mdsol/pry,deivid-rodriguez/pry,bronzdoc/pry,tdg5/pry,icebreaker/pry,jasnow/pry,scalixte-mdsol/pry,ngpestelos/pry,pvdb/pry,85x14/pry,sferik/pry,josue804/debunker,richo/pry,shockone/pry,printercu/pry,85x14/pry,printercu/pry,sideci-sample/sideci-sample-pry,ngpestelos/pry,scalixte-mdsol/pry,tdg5/pry,caporta/pry,nju520/pry,shockone/pry,raeno/pry,raeno/pry,pvdb/pry,caporta/pry,sferik/pry,ooohiroyukiooo/pry,85x14/pry,shockone/pry,icebreaker/pry,nju520/pry,jasonm23/pry,nju520/pry,jasnow/pry,jasonm23/pry,caporta/pry,gfvcastro/pry,ooohiroyukiooo/pry,raeno/pry