CI can be NULL.

Unused CI (introduced from peephole optimization, etc) can be NULL
so introduce NULL check.

compile.c

@@ -10294,6 +10294,7 @@ ibf_dump_ci_entries(struct ibf_dump *dump, const rb_iseq_t *iseq)
 
     for (i = 0; i < ci_size; i++) {
         const struct rb_callinfo *ci = cds[i].ci;
+        if (ci != NULL) {
             ibf_dump_write_small_value(dump, ibf_dump_id(dump, vm_ci_mid(ci)));
             ibf_dump_write_small_value(dump, vm_ci_flag(ci));
             ibf_dump_write_small_value(dump, vm_ci_argc(ci));
@@ -10311,6 +10312,11 @@ ibf_dump_ci_entries(struct ibf_dump *dump, const rb_iseq_t *iseq)
                 ibf_dump_write_small_value(dump, 0);
             }
         }
+        else {
+            // TODO: truncate NULL ci from call_data.
+            ibf_dump_write_small_value(dump, (VALUE)-1);
+        }
+    }
 
     return offset;
 }
@@ -10331,6 +10337,7 @@ ibf_load_ci_entries(const struct ibf_load *load,
 
     for (i = 0; i < ci_size; i++) {
         VALUE mid_index = ibf_load_small_value(load, &reading_pos);
+        if (mid_index != (VALUE)-1) {
             ID mid = ibf_load_id(load, mid_index);
             unsigned int flag = (unsigned int)ibf_load_small_value(load, &reading_pos);
             unsigned int argc = (unsigned int)ibf_load_small_value(load, &reading_pos);
@@ -10350,6 +10357,12 @@ ibf_load_ci_entries(const struct ibf_load *load,
             RB_OBJ_WRITTEN(load->iseq, Qundef, cds[i].ci);
             cds[i].cc = vm_cc_empty();
         }
+        else {
+            // NULL ci
+            cds[i].ci = NULL;
+            cds[i].cc = NULL;
+        }
+    }
 }
 
 static ibf_offset_t