check whether xform->Lut is valid in cmsTransform2DeviceLink

[x2018]

In cmsTransform2DeviceLink(), it does not check if xform -> Lut is valid, which can result in potential NULL dereference:

==27935==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000000670a92 bp 0x7fffad69b910 sp 0x7fffad69b900 T0)
==27935==The signal is caused by a READ memory access.
==27935==Hint: address points to the zero page.
    #0 0x670a92 in cmsPipelineGetPtrToFirstStage Little-CMS-lcms2.15/src/cmslut.c:1639:18
    #1 0x6de22c in cmsTransform2DeviceLink Little-CMS-lcms2.15/src/cmsvirt.c:1079:11

The xform -> Lut can be NULL as cmsCreateExtendedTransform() will return a fake transform that sets xform -> Lut to NULL, when the flag is cmsFLAGS_NULLTRANSFORM.

Note: here I suppose the valid of arguments should be checked by the caller, so I did not add checks to the related functions in cmslut.c.