tailf: Fix previously adjusted segfault patch

tailf: Fix previously adjusted segfault patch

Casting the value to be checked to size_t renders the check useless.
If st_size is SIZE_MAX+1, it will be truncated to 0 and the check
succeeds. In fact, this check can never be false because every value
stored in a size_t is smaller or equal to SIZE_MAX.

I think this adjustment was meant to fix a compiler warning for 64 bit
systems for which sizeof(off_t) is sizeof(size_t), but the signedness
differs.

Going unconditionally to the greatest possible unsigned int type if
st_size is positive (off_t is signed) will fix this issue.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>