Don't make TAB exit game if bound to inventory.

Don't make TAB exit game if bound to inventory.

I play with the TAB key bound to the inventory. However, the
code here assumes that TAB means "close formspec" in all contexts,
including the main menu. This causes my game to exit when I attempt
to TAB in between USERNAME and PASSWORD fields.

We know when m_client != NULL that the game is a client game and
not in the main menu, and then it's OK to use the INVENTORY bound
key to exit the formspec, since it's not the main menu.