Fixed an unreachable statement in launch.cpp.
Fixed an unreachable statement in launch.cpp.

The statement is actually reachable if exec fails. This patch fix it
so that it is actually unreachable.

Review: https://reviews.apache.org/r/54673

craimbert/mesos,dforsyth/mesos,yuquanshan/customizedMesos,zmalik/mesos,verizonlabs/mesos,andschwa/mesos,reneploetz/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,asamerh4/mesos,reneploetz/mesos,dforsyth/mesos,zmalik/mesos,craimbert/mesos,andschwa/mesos,asamerh4/mesos,gsantovena/mesos,jpeach/mesos,yuquanshan/customizedMesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,asamerh4/mesos,chhsia0/mesos,asamerh4/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,chhsia0/mesos,neilconway/mesos,Gilbert88/mesos,kaysoky/mesos,andschwa/mesos,Gilbert88/mesos,zmalik/mesos,neilconway/mesos,zmalik/mesos,jpeach/mesos,asamerh4/mesos,dforsyth/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos,verizonlabs/mesos,jpeach/mesos,yuquanshan/customizedMesos,chhsia0/mesos,craimbert/mesos,neilconway/mesos,gsantovena/mesos,craimbert/mesos,abudnik/mesos,Gilbert88/mesos,verizonlabs/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,zmalik/mesos,andschwa/mesos,verizonlabs/mesos,zmalik/mesos,kaysoky/mesos,reneploetz/mesos,verizonlabs/mesos,gsantovena/mesos,abudnik/mesos,kaysoky/mesos,jpeach/mesos,jpeach/mesos,dforsyth/mesos,asamerh4/mesos,gsantovena/mesos,yuquanshan/customizedMesos,chhsia0/mesos,abudnik/mesos,yuquanshan/customizedMesos,gsantovena/mesos,andschwa/mesos,abudnik/mesos,neilconway/mesos,yuquanshan/customizedMesos,neilconway/mesos,gsantovena/mesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,asamerh4/mesos,neilconway/mesos,andschwa/mesos,zmalik/mesos,neilconway/mesos,abudnik/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,reneploetz/mesos,yuquanshan/customizedMesos,jpeach/mesos,andschwa/mesos,craimbert/mesos,reneploetz/mesos,craimbert/mesos,jpeach/mesos,reneploetz/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos