optimize the code in cmsFreeProfileSequenceDescription()

[x2018]

Hi, when I test the release version 2.15, I got the error below:

==12022==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000028 (pc 0x0000006a3e15 bp 0x7ffff47d3380 sp 0x7ffff47d32c0 T0)
==12022==The signal is caused by a READ memory access.
==12022==Hint: address points to the zero page.
    #0 0x6a3e15 in cmsFreeProfileSequenceDescription Little-CMS-lcms2.15/src/cmsnamed.c:818:40
    #1 0x6a4d99 in cmsDupProfileSequenceDescription Little-CMS-lcms2.15/src/cmsnamed.c:864:5

Based on simple analysis, it is because pseq ->seq is not checked in cmsFreeProfileSequenceDescription() of version 2.15.
I found that the above problem is fixed in ac9e8e1, but the code could still be optimized:
if pseq ->seq is NULL, then it is unnecessary to enter the loop so that we just need to check the pointer one time.