Fix comparison of Header.MipLengths

[ryandesign]

Fixes compile warning:

warning: comparison of array 'Header.MipLengths' equal to a null pointer is always false [-Wtautological-pointer-compare]
                                        if (Header.MipOffsets[Mip] == 0 || Header.MipLengths == 0)  // No more mipmaps in the file.
                                                                           ~~~~~~~^~~~~~~~~~    ~