Fix a debugging messages in the formatRenderedProblem method.

The `formatRenderedProblem` method of the `FormatRenderedProblem` module
ensures that the `debug_messages` key of the rendered problem result is
an array reference, but checks the wrong thing.  It checks if the result
itself is an array reference instead of the `debug_messages` hash key of
the result.  Since the result is always a hash reference and never an
array reference, this means that the debugging messages from PG are
always wiped out and replaced with a reference to an empty array.

This was discovered when investigating #2661.
If we want to switch the PG problem editor to using the `debug` output
format of the `formatRenderedProblem` method we could, but in order to
get the benefit of that change we need this.

lib/FormatRenderedProblem.pm

@@ -188,7 +188,7 @@ sub formatRenderedProblem {
 	}
 
 	# Make sure this is defined and is an array reference as saveGradeToLTI might add to it.
-	$rh_result->{debug_messages} = [] unless defined $rh_result && ref $rh_result eq 'ARRAY';
+	$rh_result->{debug_messages} = [] unless ref $rh_result->{debug_messages} eq 'ARRAY';
 
 	$forbidGradePassback = 1 if !$forbidGradePassback && !$submitMode;