Don't crash on imported photo: Bug #734986

The user's photo was identified as JFIF but is an incomplete file.
The JFIF sniffer reported it could not identify the file, which is
somewhat incorrect.  The RAW sniffer then tried to identify the file
and crashed inside of libraw.

Now when a sniffer can appropriately detect the file is corrupt it
will be reported as such.  All GdkSniffers will do this if they (a)
read the entire file and (b) no image was prepared.  Because they all
use pre-condition checking to sanity check that the file is their
format (via magic header bytes), this works.