Verify first 4 bytes of TIFF file

This change prevents libvips' TIFF loader attempting to handle files in the `.orf` Olympus Raw format, which happen to share their first two bytes with the TIFF format. I suspect there may be other formats guilty of this orf-ul crime.

Before:
```
curl -O http://sample-images.metadata-extractor.googlecode.com/git/Olympus%20Pen%20E-PL1.orf
$ vips im_copy Olympus%20Pen%20E-PL1.orf test.jpg
Olympus%20Pen%20E-PL1.orf: Not a TIFF file, bad version number 20306 (0x4f52)
tiff2vips: unable to open "Olympus%20Pen%20E-PL1.orf" for input
```

After:
```
$ vips im_copy Olympus%20Pen%20E-PL1.orf test.jpg
VipsFormat: file "Olympus%20Pen%20E-PL1.orf" not a known format
```

The version of libmagick I'm using doesn't seem to recognise this sample `.orf` file either, but at least it's now given the chance.