MDL-68047 tcpdf: Prevent images out from cache to be removed

This corresponds to https://github.com/tecnickcom/TCPDF/pull/174
(and was fixed bycommit 510070b) and was a regression causing
image files to be removed unconditionally. Only if the images
are from cache, they can be removed.

It's fixed upstream in version 6.3.4 and up.

lib/tcpdf/readme_moodle.txt

@@ -9,6 +9,7 @@ Description of TCPDF library import 6.3.2
   * 18dabac - https://git.io/JeKfU
   * 60c9db7 - https://git.io/JeKfT
   * 1adcd76 - https://git.io/JeKft
+  * 5e4d999 - https://git.io/JvrxB
 * visit http://127.0.0.1/lib/tests/other/pdflibtestpage.php and view the pdf
 
 Important

lib/tcpdf/tcpdf.php

@@ -7798,7 +7798,9 @@ class TCPDF {
 			}
 			if (isset($this->imagekeys)) {
 				foreach($this->imagekeys as $file) {
-					unlink($file);
+					if (strpos($file, K_PATH_CACHE) === 0) {
+						unlink($file);
+					}
 				}
 			}
 		}