Fix guestfs___rollback_cmdline when pos == 0.

Fix guestfs___rollback_cmdline when pos == 0.

Because of the previous change to size_t, when pos == 0 the loop would
wrap around and cause a segmentation fault.

This fixes a regression introduced by
commit 10167cea98f93a74abe63f0a54d3a662997e7489.
(cherry picked from commit 607aa95de927b5b8e4b212655c71bf21acbba372)