lua: lua_tostring() on an error value can return NULL

Lua is so clever it allows values that can't be converted to strings, in
which case lua_tostring() returns NULL. Trigger undefined behavior.

1 files changed, 4 insertions, 2 deletions

diff --git a/player/lua.c b/player/lua.c
index 791a7d4fea..80d40e1b6a 100644
--- a/player/lua.c
+++ b/player/lua.c
@@ -346,8 +346,10 @@ static int run_lua(lua_State *L)
     // run this under an error handler that can do backtraces
     lua_pushcfunction(L, error_handler); // errf
     lua_pushcfunction(L, load_scripts); // errf fn
-    if (lua_pcall(L, 0, 0, -2)) // errf [error]
-        MP_FATAL(ctx, "Lua error: %s\n", lua_tostring(L, -1));
+    if (lua_pcall(L, 0, 0, -2)) { // errf [error]
+        const char *e = lua_tostring(L, -1);
+        MP_FATAL(ctx, "Lua error: %s\n", e ? e : "(unknown)");
+    }
 
     return 0;
 }