[Prevent NULL-ptr dereference] exit early if failed to read coefficients

[the-shank]

jpeg_read_coefficients() can return NULL under certain conditions. When this happens, the program would segfault later during the execution.

This fix checks if the return value of jpeg_read_coefficients() is NULL and exits if its NULL.

[tjko]

Thanks! I modified your approach slightly, so processing is not aborted, but simply skip the "broken" file, if this happens.

Does this fix the issue you noticed? 240a11526ec06f8f9e7a7dfc275bd0862f182c0d

[the-shank]

Yes, this would fix the issue as well. Thanks!