luv_cfpcall: Fix stack balancing after an uncaught error

The default error handling code for printing uncaught errors uses
luaL_tolstring(). This returns the resulting string, but also pushes it onto
the stack. After we have printed it, it is safe to pop from the stack.

Leaving it on the stack causes the stack to be unbalanced.

Fixes #735 (where a test case can be found)

src/luv.c

@@ -726,9 +726,11 @@ LUALIB_API int luv_cfpcall(lua_State* L, int nargs, int nresult, int flags) {
   case LUA_ERRRUN:
   case LUA_ERRERR:
   default:
-    if ((flags & LUVF_CALLBACK_NOERRMSG) == 0)
+    if ((flags & LUVF_CALLBACK_NOERRMSG) == 0) {
       fprintf(stderr, "Uncaught Error: %s\n",
               luaL_tolstring(L, lua_absindex(L, -1), NULL));
+      lua_pop(L, 1); // Remove error string pushed by luaL_tolstring()
+    }
     if ((flags & LUVF_CALLBACK_NOEXIT) == 0)
       exit(-1);
     lua_pop(L, 1);