PluginOutput: Return correctly cut off output

If the output contains a cut off HTML element, it merges the next list-item into the same tag, because the end tag is missing.
In this case, the output must cut off up to the last valid string(part).

library/Icingadb/Util/PluginOutput.php

@@ -202,8 +202,14 @@ public function render()
             $output = $this->processHtml($output);
         }
 
-        if ($this->characterLimit) {
+        if (strlen($output) > $this->characterLimit) {
             $output = substr($output, 0, $this->characterLimit);
+
+            // Remove the last output/html element if it is cut off to display only valid content
+            $endAt = strrpos($output, '\\_');
+            if ($endAt !== false) {
+                $output = rtrim(substr($output, 0, $endAt));
+            }
         }
 
         $this->renderedOutput = $output;