Fix Value::String::to_pointer for Lua < 5.4

Author: khvzak

src/value.rs

@@ -129,9 +129,14 @@ impl Value {
     #[inline]
     pub fn to_pointer(&self) -> *const c_void {
         match self {
+            Value::String(String(vref)) => {
+                // In Lua < 5.4 (excluding Luau), string pointers are NULL
+                // Use alternative approach
+                let lua = vref.lua.lock();
+                unsafe { ffi::lua_tostring(lua.ref_thread(), vref.index) as *const c_void }
+            }
             Value::LightUserData(ud) => ud.0,
-            Value::String(String(vref))
-            | Value::Table(Table(vref))
+            Value::Table(Table(vref))
             | Value::Function(Function(vref))
             | Value::Thread(Thread(vref, ..))
             | Value::UserData(AnyUserData(vref))