A fix and a test case for Bug#1061147 "tonumber64 is not transitive"

https://bugs.launchpad.net/tarantool/+bug/1061147

Patch contributed by Mons Anderson

When accessing Lua stack via a numeric index, first calculate
absolute value of the index.

src/tarantool_lua.m

@@ -126,6 +126,9 @@ tarantool_lua_tointeger64(struct lua_State *L, int idx)
 	}
 	case LUA_TCDATA:
 	{
+		/* Calculate absolute value in the stack. */
+		if (idx < 0)
+			idx = lua_gettop(L) + idx + 1;
 		GCcdata *cd = cdataV(L->base + idx - 1);
 		if (cd->typeid != CTID_INT64 && cd->typeid != CTID_UINT64) {
 			luaL_error(L,

test/box/lua.test

@@ -573,3 +573,7 @@ print """# A test case for bitwise operations """
 exec admin "lua bit.lshift(1, 32)"
 exec admin "lua bit.band(1, 3)"
 exec admin "lua bit.bor(1, 2)"
+
+print """# A test case for Bug#1061747 'tonumber64 is not transitive'"""
+exec admin "lua tonumber64(tonumber64(2))"
+exec admin "lua tostring(tonumber64(tonumber64(3)))"