Add additional error checking to label handling (#1907)

CDash currently fails during the submission process if a label is
applied to something other than a build failure. This PR adds additional
checking, which allows the submission process to continue when labels
are applied to invalid elements.

app/cdash/xml_handlers/build_handler.php

@@ -280,7 +280,7 @@ public function endElement($parser, $name)
         } elseif ($name == 'LABEL' && $parent == 'LABELS') {
             if (!empty($this->ErrorSubProjectName)) {
                 $this->SubProjectName = $this->ErrorSubProjectName;
-            } elseif (isset($this->Error)) {
+            } elseif (isset($this->Error) && $this->Error instanceof BuildFailure) {
                 $this->Error->AddLabel($this->Label);
             } else {
                 $this->Labels[] = $this->Label;