Fix 500 error for invalid path when visiting blame page

Closes #25761.

app/controllers/projects/blame_controller.rb

@@ -8,6 +8,9 @@ class Projects::BlameController < Projects::ApplicationController
 
   def show
     @blob = @repository.blob_at(@commit.id, @path)
+    
+    return render_404 unless @blob
+
     @blame_groups = Gitlab::Blame.new(@blob, @commit).groups
   end
 end

changelogs/unreleased/fix-blame-500.yml

@@ -0,0 +1,4 @@
+---
+title: Fix blame 500 error on invalid path.
+merge_request: 25761
+author: Jeff Stubler

spec/controllers/projects/blame_controller_spec.rb

@@ -25,5 +25,10 @@ describe Projects::BlameController do
       let(:id) { 'master/files/ruby/popen.rb' }
       it { is_expected.to respond_with(:success) }
     end
+
+    context "invalid file" do
+      let(:id) { 'master/files/ruby/missing_file.rb'}
+      it { expect(response).to have_http_status(404) }
+    end
   end
 end