Fix throw missing file error (#6)

* Update AppleSafariPinGenerator.php

Fix throwing error for non-existent file when the conversion didn't work, instead of the thrown error

* Change error suppression to if statement

Author: skeemer

src/AppleSafariPinGenerator.php

@@ -68,7 +68,9 @@ public function tempFile(callable $callback, string $prefix = 'favicon-tmp')
             return $callback($tempSource, $tempTarget);
         } finally {
             \unlink($tempSource);
-            \unlink($tempTarget);
+            if (\is_file($tempTarget) {
+                \unlink($tempTarget);
+            }
         }
     }