tap: check for invalid fetched tap name.
tap: check for invalid fetched tap name.

Fixes #7732.

konqui/brew,JCount/brew,EricFromCanada/brew,claui/brew,EricFromCanada/brew,JCount/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,Homebrew/brew,claui/brew,vitorgalvao/brew,JCount/brew,Homebrew/brew,JCount/brew,nandub/brew,sjackman/homebrew,Homebrew/brew,mahori/brew,Homebrew/brew,nandub/brew,MikeMcQuaid/brew,reitermarkus/brew,mahori/brew,nandub/brew,EricFromCanada/brew,sjackman/homebrew,EricFromCanada/brew,claui/brew,sjackman/homebrew,mahori/brew,mahori/brew,vitorgalvao/brew,reitermarkus/brew,MikeMcQuaid/brew,sjackman/homebrew,vitorgalvao/brew,konqui/brew,nandub/brew,reitermarkus/brew,vitorgalvao/brew,claui/brew,reitermarkus/brew,konqui/brew,konqui/brew