Fixed LuaJIT when not compiled with COMPAT mode (#6605)

LuaJIT as installed by apt doesn't include the COMPAT mode for using
Lua5.2 features. So this change just makes the flatbuffer code use a
common implementation.

tests/LuaTest.sh

@@ -19,7 +19,13 @@ test_dir="$(pwd)"
 
 ${test_dir}/../flatc --lua -I include_test monster_test.fbs
 
-echo "Run with LuaJIT:"
-luajit luatest.lua
-echo "Run with Lua 5.3:"
-lua5.3 luatest.lua
+declare -a versions=(luajit lua5.3)
+
+for i in "${versions[@]}"
+do
+    if command -v $i &> /dev/null
+    then
+        echo "[$i]"
+        $i luatest.lua
+    fi
+done

tests/luatest.lua

@@ -1,4 +1,5 @@
 package.path = string.format("../lua/?.lua;./?.lua;%s",package.path)
+local compat = require("flatbuffers.compat")
 
 local performBenchmarkTests = false
 
@@ -20,7 +21,7 @@ local function checkReadBuffer(buf, offset, sizePrefix)
     
     if sizePrefix then               
         local size = flatbuffers.N.Int32:Unpack(buf, offset)
-        assert(size == #buf - offset - 4)
+        assert(size == buf.size - offset - 4)
         offset = offset + flatbuffers.N.Int32.bytewidth
     end