clean up zero-byte logfile on immediate exit

clean up zero-byte logfile on immediate exit

Otherwise, a zero-byte log file stays behind after every call to `i3
--get-socketpath`. Also, replace "return" calls with more explicit "exit"
calls.

Before:

$ ls -ld /tmp/i3* | wc -l; \
  repeat 10 i3 --get-socketpath >/dev/null; \
  ls -ld /tmp/i3* | wc -l
1
11

Now:

$ ls -ld /tmp/i3* | wc -l; \
  repeat 10 i3 --get-socketpath >/dev/null; \
  ls -ld /tmp/i3* | wc -l
1
1

Signed-off-by: Julius Plenz <julius@plenz.com>