Lock only active_units references

556a728508 was not good maybe because it
wasn't using list_for_each_safe. If list_for_each_safe is safe for
list_del for any nodes (is that true?), this should be fine.

mjit_worker.c

@@ -947,7 +947,8 @@ compile_compact_jit_code(char* c_file)
     // compacted functions (not done yet).
     bool success = true;
     CRITICAL_SECTION_START(3, "before active_units list_for_each");
-    list_for_each(&active_units.head, child_unit, unode) {
+    list_for_each_safe(&active_units.head, child_unit, next, unode) {
+        CRITICAL_SECTION_FINISH(3, "after active_units list_for_each");
         char funcname[MAXPATHLEN];
         sprint_funcname(funcname, child_unit);
 
@@ -961,6 +962,8 @@ compile_compact_jit_code(char* c_file)
         if (!iseq_label) iseq_label = sep = "";
         fprintf(f, "\n/* %s%s%s:%ld */\n", iseq_label, sep, iseq_path, iseq_lineno);
         success &= mjit_compile(f, child_unit->iseq, funcname, child_unit->id);
+
+        CRITICAL_SECTION_START(3, "before active_units list_for_each");
     }
     CRITICAL_SECTION_FINISH(3, "after active_units list_for_each");