utils: skip migration of broken formulae symlinks.
utils: skip migration of broken formulae symlinks.

Closes https://github.com/Homebrew/homebrew-core/issues/4973

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