Fix calculating average color for *.ico

Currently colorPalette() always fails for *.ico due to a logic error.
It's a regression from 8f749fe61b.

include/colors.php

@@ -355,8 +355,9 @@ function colorPalette(string $imageFile, int $numColors, int $granularity = 5):
 			} else {
 				$img = @$ico->images[count($ico->images)-1]->getImageResource();
 			}
+		} else {
+			return null;
 		}
-		return null;
 	} else if ($size[0] > 0 && $size[1] > 0) {
 		$img = @imagecreatefromstring(file_get_contents($imageFile));
 	}