Don't call malloc with 0

It seems not-uncommon for methods to have no IV, ISE, or ICVARC caches.
Calling malloc with 0 will actually allocate something, so if there
aren't any caches (`ISEQ_IS_SIZE(body) == 0`), then we can avoid
allocating memory by not calling malloc.  If there are no caches, then
theoretically nobody should be reading from the buffer anyway.

This saves about 1MB on Lobsters benchmark.

compile.c

@@ -2421,7 +2421,12 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
     generated_iseq = ALLOC_N(VALUE, code_index);
     insns_info = ALLOC_N(struct iseq_insn_info_entry, insn_num);
     positions = ALLOC_N(unsigned int, insn_num);
-    body->is_entries = ZALLOC_N(union iseq_inline_storage_entry, ISEQ_IS_SIZE(body));
+    if (ISEQ_IS_SIZE(body)) {
+        body->is_entries = ZALLOC_N(union iseq_inline_storage_entry, ISEQ_IS_SIZE(body));
+    }
+    else {
+        body->is_entries = NULL;
+    }
     body->call_data = ZALLOC_N(struct rb_call_data, body->ci_size);
     ISEQ_COMPILE_DATA(iseq)->ci_index = 0;
 
@@ -2560,8 +2565,8 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
                         case TS_CALLDATA:
                         {
                             const struct rb_callinfo *source_ci = (const struct rb_callinfo *)operands[j];
-                            struct rb_call_data *cd = &body->call_data[ISEQ_COMPILE_DATA(iseq)->ci_index++];
                             assert(ISEQ_COMPILE_DATA(iseq)->ci_index <= body->ci_size);
+                            struct rb_call_data *cd = &body->call_data[ISEQ_COMPILE_DATA(iseq)->ci_index++];
                             cd->ci = source_ci;
                             cd->cc = vm_cc_empty();
                             generated_iseq[code_index + 1 + j] = (VALUE)cd;
@@ -12297,7 +12302,13 @@ ibf_load_iseq_each(struct ibf_load *load, rb_iseq_t *iseq, ibf_offset_t offset)
     load_body->icvarc_size          = icvarc_size;
     load_body->ise_size             = ise_size;
     load_body->ic_size              = ic_size;
-    load_body->is_entries           = ZALLOC_N(union iseq_inline_storage_entry, ISEQ_IS_SIZE(load_body));
+
+    if (ISEQ_IS_SIZE(load_body)) {
+        load_body->is_entries       = ZALLOC_N(union iseq_inline_storage_entry, ISEQ_IS_SIZE(load_body));
+    }
+    else {
+        load_body->is_entries       = NULL;
+    }
                                       ibf_load_ci_entries(load, ci_entries_offset, ci_size, &load_body->call_data);
     load_body->outer_variables      = ibf_load_outer_variables(load, outer_variables_offset);
     load_body->param.opt_table      = ibf_load_param_opt_table(load, param_opt_table_offset, param_opt_num);