Another check to cgats

Again, pointless but for fuzzer hapiness.

src/cmscgats.c

@@ -1671,7 +1671,7 @@ char* GetData(cmsIT8* it8, int nSet, int nField)
     int nSamples    = t -> nSamples;
     int nPatches    = t -> nPatches;
 
-    if (nSet >= nPatches || nField >= nSamples)
+    if (nSet < 0 || nSet >= nPatches || nField < 0 || nField >= nSamples)
         return NULL;
 
     if (!t->Data) return NULL;