Cleanup: Add assert for customdata realloc size

This gives a more clear error than finding the error with the signed
to unsigned conversion for size_t.

source/blender/blenkernel/intern/customdata.cc

@@ -2329,6 +2329,7 @@ bool CustomData_merge(const CustomData *source,
 
 void CustomData_realloc(CustomData *data, int totelem)
 {
+  BLI_assert(totelem > 0);
   for (int i = 0; i < data->totlayer; i++) {
     CustomDataLayer *layer = &data->layers[i];
     const LayerTypeInfo *typeInfo;