Prevent underflow of union padding

src/llvm_backend_const.cpp

@@ -886,6 +886,8 @@ gb_internal lbValue lb_const_value(lbModule *m, Type *type, ExactValue value, lb
 				return res;
 			}
 		} else {
+			// NOTE(korvahkh): forces calculation of variant_block_size
+			type_size_of(bt);
 			i64 block_size = bt->Union.variant_block_size;
 
 			lbValue cv = lb_const_value(m, value_type, value, cc);