audit: fix keg_only check

[jonchang]

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Fixes #7341

Before this patch:

% brew audit --strict --new-formula xsimd   
Error: undefined method `provided_by_macos?' for nil:NilClass
Please report this issue:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:406:in `block (2 levels) in audit_deps'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/delegate.rb:349:in `each'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/delegate.rb:349:in `block in delegating_block'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:378:in `block in audit_deps'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:375:in `each'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:375:in `audit_deps'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:1017:in `block in audit'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:1010:in `each'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:1010:in `audit'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:117:in `block in audit'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:110:in `each'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/audit.rb:110:in `audit'
/usr/local/Homebrew/Library/Homebrew/brew.rb:110:in `<main>'

After this patch:

% brew audit --strict --new-formula xsimd
xsimd:
  * New formulae in homebrew/core should not have a `bottle do` block
Error: 1 problem in 1 formula detected

[MikeMcQuaid]

Oops, thanks for quick fix @jonchang.