Add MiqReportResult#valid_report_column?

https://bugzilla.redhat.com/show_bug.cgi?id=1590908

A quicker check than having to use `MiqReportResult#report_results`,
since binary_blob doesn't need to be loaded into memory.  On top of
that, generally the binary_blob.data isn't even used in the controller
at the end of the day, and this version of the MiqReport record ends up
being what is used when generating the page for viewing.

app/models/miq_report_result.rb

@@ -53,6 +53,13 @@ def status_message
     miq_task.nil? ? _("The task associated with this report is no longer available") : miq_task.message
   end
 
+  # Use this method over `report_results` if you don't plan on using the
+  # `binary_blob` associated with the MiqReportResult (chances are you don't
+  # need it).
+  def valid_report_column?
+    report.kind_of?(MiqReport)
+  end
+
   def report_results
     if binary_blob
       data = binary_blob.data