installer: don't try and unlink nil tabfile.
installer: don't try and unlink nil tabfile.

Closes Homebrew/homebrew#23472.

nandub/brew,ilovezfs/brew,konqui/brew,amar-laksh/brew_sudo,muellermartin/dist,EricFromCanada/brew,staticfloat/brew,pseudocody/brew,JCount/brew,staticfloat/brew,rwhogg/brew,claui/brew,toonetown/homebrew,mahori/brew,toonetown/homebrew,mgrimes/brew,mgrimes/brew,zmwangx/brew,ilovezfs/brew,reitermarkus/brew,AnastasiaSulyagina/brew,JCount/brew,maxim-belkin/brew,sjackman/homebrew,pseudocody/brew,muellermartin/dist,alyssais/brew,vitorgalvao/brew,tonyg/homebrew,DomT4/brew,mistydemeo/homebrew,zmwangx/brew,jmsundar/brew,mistydemeo/homebrew,reitermarkus/brew,claui/brew,claui/brew,hanxue/linuxbrew,toonetown/homebrew,aw1621107/brew,MikeMcQuaid/brew,mgrimes/brew,vitorgalvao/brew,vitorgalvao/brew,konqui/brew,DomT4/brew,vitorgalvao/brew,jmsundar/brew,staticfloat/brew,aw1621107/brew,konqui/brew,tonyg/homebrew,EricFromCanada/brew,amar-laksh/brew_sudo,konqui/brew,hanxue/linuxbrew,mahori/brew,JCount/brew,Linuxbrew/brew,sjackman/homebrew,maxim-belkin/brew,gordonmcshane/homebrew,maxim-belkin/brew,reelsense/brew,nandub/brew,reitermarkus/brew,AnastasiaSulyagina/brew,amar-laksh/brew_sudo,bfontaine/brew,MikeMcQuaid/brew,hanxue/linuxbrew,gregory-nisbet/brew,Homebrew/brew,reelsense/brew,JCount/brew,sjackman/homebrew,AnastasiaSulyagina/brew,Homebrew/brew,sjackman/homebrew,nandub/brew,alyssais/brew,EricFromCanada/brew,Homebrew/brew,reelsense/brew,pseudocody/brew,rwhogg/brew,Linuxbrew/brew,palxex/brew,EricFromCanada/brew,bfontaine/brew,jmsundar/brew,alyssais/brew,DomT4/brew,nandub/brew,mahori/brew,Linuxbrew/brew,rwhogg/brew,Homebrew/brew,palxex/brew,tonyg/homebrew,gordonmcshane/homebrew,Linuxbrew/brew,bfontaine/brew,mahori/brew,MikeMcQuaid/brew,gordonmcshane/homebrew,DomT4/brew,muellermartin/dist,claui/brew,palxex/brew,gregory-nisbet/brew,gregory-nisbet/brew,zmwangx/brew,ilovezfs/brew,mistydemeo/homebrew,MikeMcQuaid/brew,reitermarkus/brew,aw1621107/brew