Move reserved handle check to C++ too

This allows to completely eliminate statements like `val v;` as compiler can statically prove it doesn't need _emval_free.

Author: RReverser

src/embind/emval.js

@@ -80,9 +80,7 @@ var LibraryEmVal = {
 
   _emval_free__deps: ['$emval_handles'],
   _emval_free: (handle) => {
-    if (handle >= emval_handles.reserved) {
-      emval_handles.free(handle);
-    }
+    emval_handles.free(handle);
   },
 
   _emval_run_destructors__deps: ['_emval_free', '$Emval', '$runDestructors'],

system/include/emscripten/val.h

@@ -38,9 +38,10 @@ void _emval_register_symbol(const char*);
 
 enum {
   _EMVAL_UNDEFINED = 1,
-  _EMVAL_NULL = 2,
-  _EMVAL_TRUE = 3,
-  _EMVAL_FALSE = 4
+  _EMVAL_NULL,
+  _EMVAL_TRUE,
+  _EMVAL_FALSE,
+  _EMVAL_UNRESERVED_START
 };
 
 typedef struct _EM_DESTRUCTORS* EM_DESTRUCTORS;
@@ -322,7 +323,9 @@ struct val_metadata {
 
   // should be only accessible from dec_ref
   ~val_metadata() {
-    _emval_free(handle);
+    if (handle >= EM_VAL(internal::_EMVAL_UNRESERVED_START)) {
+      internal::_emval_free(handle);
+    }
   }
 };