Fix unsigned < 0 check

Two places in malware.c were using `fsize`, defined as `unsigned int`,
to receive the result of `lseek()` and then checking if the value was
less than 0.  As clang says:

```
malware.c:1228:46: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
          if ((fsize = lseek(clam_fd, 0, SEEK_END)) < 0) {
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~
```

Fix.  Use `off_t`, which we're already using elsewhere, then use
`fsize_uint` to handle off_t being potentially 64-bit, and a
sanity-check on conversion which hopefully won't be optimised away by
compilers.