safe_create_leading_directories(): always restore slash at end of loop

safe_create_leading_directories(): always restore slash at end of loop

Always restore the slash that we scribbled over at the end of the
loop, rather than also fixing it up at each premature exit from the
loop.  This makes it harder to forget to do the cleanup as new paths
are added to the code.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>