[CORE] Fix undefined offset warnings in different files

This problem was presentend in the following issue:
https://notabug.org/diogo/gnu-social/issues/60

AcceptHeader/util:
- Perform isset before using the required array values

extlib/AcceptHeader.php

@@ -49,12 +49,18 @@ class AcceptHeader extends \ArrayObject
         foreach ($items as $item) {
             $elems = explode(';', $item);
 
-            $acceptElement = [];
-            $mime = current($elems);
-            list($type, $subtype) = explode('/', $mime);
-            $acceptElement['type'] = trim($type);
-            $acceptElement['subtype'] = trim($subtype);
-            $acceptElement['raw'] = $mime;
+            $mime  = current($elems);
+            $types = explode('/', $mime);
+
+            if (!isset($types[1])) {
+                continue;
+            }
+
+            $acceptElement = [
+                'raw'     => $mime,
+                'type'    => trim($types[0]),
+                'subtype' => trim($types[1]),
+            ];
 
             $acceptElement['params'] = [];
             while (next($elems)) {