Gracefully handle references with null bytes

`Rugged::Reference.valid_name?` used in
`Gitlab::GitRefValidator.validate` fails on strings containing null
bytes because it uses `StringValueCStr()`. Per
https://silverhammermba.github.io/emberb/c/:

Ruby’s String kinda corresponds to C’s char*. The simplest macro is
StringValueCStr() which returns a null-terminated char* for a
String. The problem here is that a Ruby String might contain nulls - in
which case StringValueCStr() will raise an ArgumentError!

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54466

changelogs/unreleased/sh-handle-string-null-bytes.yml

+---
+title: Gracefully handle references with null bytes
+merge_request: 23365
+author:
+type: fixed

lib/gitlab/git_ref_validator.rb

@@ -13,7 +13,11 @@ module Gitlab
       return false if ref_name.start_with?(*not_allowed_prefixes)
       return false if ref_name == 'HEAD'
 
-      Rugged::Reference.valid_name? "refs/heads/#{ref_name}"
+      begin
+        Rugged::Reference.valid_name?("refs/heads/#{ref_name}")
+      rescue ArgumentError
+        return false
+      end
     end
   end
 end

spec/lib/gitlab/git_ref_validator_spec.rb

@@ -27,4 +27,5 @@ describe Gitlab::GitRefValidator do
   it { expect(described_class.validate('-branch')).to be_falsey }
   it { expect(described_class.validate('.tag')).to be_falsey }
   it { expect(described_class.validate('my branch')).to be_falsey }
+  it { expect(described_class.validate("\xA0\u0000\xB0")).to be_falsey }
 end