tap-new: restrict new tap names.

Don't want to restrict this for all taps otherwise existing ones may
explode.

Fixes #7734

Library/Homebrew/dev-cmd/tap-new.rb

@@ -22,7 +22,10 @@ module Homebrew
   def tap_new
     tap_new_args.parse
 
+    tap_name = args.named.first
     tap = Tap.fetch(args.named.first)
+    raise "Invalid tap name '#{tap_name}'" unless tap.path.to_s.match?(HOMEBREW_TAP_PATH_REGEX)
+
     titleized_user = tap.user.dup
     titleized_repo = tap.repo.dup
     titleized_user[0] = titleized_user[0].upcase

Library/Homebrew/tap.rb

@@ -34,10 +34,7 @@ class Tap
     return CoreTap.instance if ["Homebrew", "Linuxbrew"].include?(user) && ["core", "homebrew"].include?(repo)
 
     cache_key = "#{user}/#{repo}".downcase
-    tap = cache.fetch(cache_key) { |key| cache[key] = Tap.new(user, repo) }
-    raise "Invalid tap name '#{args.join("/")}'" unless tap.path.to_s.match?(HOMEBREW_TAP_PATH_REGEX)
-
-    tap
+    cache.fetch(cache_key) { |key| cache[key] = Tap.new(user, repo) }
   end
 
   def self.from_path(path)