Avoid segfault when ticky file argument is stderr

Avoid segfault when ticky file argument is stderr

If you compiled a program with -ticky and ran it with:
./foo +RTS -rstderr -RTS
the result would be a segfault. This was because the RTS interprets stderr to
mean "use debugBelch to print out messages," and sets the ticky file pointer
to NULL as a result, but PrintTickyInfo (the function in Ticky.c that prints
out the ticky report) wasn't checking for NULL.

I changed PrintTickyInfo to check whether the ticky file pointer is NULL and
output to stderr if so.

Also removed an unused import from CodeOutput.lhs.