Fix loading bytecode on big endian systems

We used to check the value before it got endian-switched
by validate_operand. Move the check after the switch and
fix the offset to match.

src/core/validation.c

@@ -406,9 +406,9 @@ static void validate_operands(Validator *val) {
         case MVM_OP_wval:
         case MVM_OP_wval_wide: {
             validate_reg_operand(val, operands[0]);
-            if (GET_UI16(val->cur_op, 0) >= val->cu->body.num_scs)
-                fail(val, MSG(val, "out of range SC index %u"), GET_UI16(val->cur_op, 0));
             validate_literal_operand(val, operands[1]);
+            if (GET_UI16(val->cur_op, -2) >= val->cu->body.num_scs)
+                fail(val, MSG(val, "out of range SC index %u"), GET_UI16(val->cur_op, -2));
             validate_literal_operand(val, operands[2]);
             break;
         }