Fix crash when RGENGC_CHECK_MODE=2

Commit dba61f4 fixes a crash when GC'ing a iseq that failed to compile.
However, if we turn on RGENGC_CHECK_MODE then rb_iseq_memsize crashes
since it cannot handle an iseq without is_entries.

iseq.c

@@ -515,6 +515,7 @@ rb_iseq_memsize(const rb_iseq_t *iseq)
         /* body->is_entries */
         size += ISEQ_IS_SIZE(body) * sizeof(union iseq_inline_storage_entry);
 
+        if (ISEQ_BODY(iseq)->is_entries) {
             /* IC entries constant segments */
             for (unsigned int ic_idx = 0; ic_idx < body->ic_size; ic_idx++) {
                 IC ic = &ISEQ_IS_IC_ENTRY(body, ic_idx);
@@ -525,6 +526,7 @@ rb_iseq_memsize(const rb_iseq_t *iseq)
                 }
                 size += sizeof(ID); // null terminator
             }
+        }
 
         /* body->call_data */
         size += body->ci_size * sizeof(struct rb_call_data);