Fix profiler sometimes not adding the "In native code" placeholder

[ivoanjo]

What does this PR do?:

In #2583 we added a very small check to our custom ddtrace_rb_profile_frames to match a similar fix added to upstream Ruby.

But I've just spotted this caused one of the profiling tests to become flaky:

$ bundle exec rspec spec/datadog/profiling --seed 14514

Failures:

  1) Datadog::Profiling::Collectors::Stack when sampling a thread with empty locations gathers a one-element stack with a "In native code" placeholder
     Failure/Error: expect(gathered_stack).to contain_exactly({ base_label: '', path: 'In native code', lineno: 0 })

       expected collection contained:  [{:base_label=>"", :lineno=>0, :path=>"In native code"}]
       actual collection contained:    []
       the missing elements were:      [{:base_label=>"", :lineno=>0, :path=>"In native code"}]
     # ./spec/datadog/profiling/collectors/stack_spec.rb:373:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:220:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:112:in `block (2 levels) in <top (required)>'
     # /home/ivo.anjo/.rvm/gems/ruby-2.7.7/gems/webmock-3.13.0/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'

  2) Datadog::Profiling::Collectors::Stack when sampling a thread with empty locations when marking sample as being in garbage collection gathers a two-element stack with a placeholder for "In native code" and another for garbage collection
     Failure/Error:
       expect(gathered_stack).to contain_exactly(
         { base_label: '', path: 'Garbage Collection', lineno: 0 },
         { base_label: '', path: 'In native code', lineno: 0 }
       )

       expected collection contained:  [{:base_label=>"", :lineno=>0, :path=>"Garbage Collection"}, {:base_label=>"", :lineno=>0, :path=>"In native code"}]
       actual collection contained:    [{:base_label=>"", :lineno=>0, :path=>"Garbage Collection"}]
       the missing elements were:      [{:base_label=>"", :lineno=>0, :path=>"In native code"}]
     # ./spec/datadog/profiling/collectors/stack_spec.rb:380:in `block (4 levels) in <top (required)>'
     # ./spec/spec_helper.rb:220:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:112:in `block (2 levels) in <top (required)>'
     # /home/ivo.anjo/.rvm/gems/ruby-2.7.7/gems/webmock-3.13.0/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'

Finished in 13.42 seconds (files took 0.44192 seconds to load)
563 examples, 2 failures, 15 pending

Failed examples:

rspec ./spec/datadog/profiling/collectors/stack_spec.rb:372 # Datadog::Profiling::Collectors::Stack when sampling a thread with empty locations gathers a one-element stack with a "In native code" placeholder
rspec ./spec/datadog/profiling/collectors/stack_spec.rb:379 # Datadog::Profiling::Collectors::Stack when sampling a thread with empty locations when marking sample as being in garbage collection gathers a two-element stack with a placeholder for "In native code" and another for garbage collection

This issue was caused by the cfp == NULL check being added before the end_cfp == NULL check we have to detect when we should add the "In native code" placeholder.

In some cases, cfp is set and the placeholder is added, but in others, it's not.

To fix this, I've swapped the order of the checks, so that the check to introduce the placeholder gets higher priority.

Motivation:

Fix bug (that also causes flaky tests).

Additional Notes:

(N/A)

How to test the change?:

Validate that running profiling test suite works with for instance seed 14154.