Check for _error in getting metadata array in PNGHandler

Before I039785d5b6, it would just suppress errors and return empty value
in which shorten out. Now, it returns [ '_error' => some value ] which
is not empty value but doesn't have anything this method wants either.

Bug: T285431
Change-Id: Ia2bc0982ffaeda0575af1481f9b84faad7d784ad

includes/media/PNGHandler.php

@@ -150,7 +150,11 @@ class PNGHandler extends BitmapHandler {
 
 		$metadata = $image->getMetadataArray();
 
-		if ( !$metadata || $metadata['frameCount'] <= 0 ) {
+		if (
+			!$metadata ||
+			( isset( $metadata['_error'] ) && $metadata['_error'] ) ||
+			$metadata['frameCount'] <= 0
+		) {
 			return $original;
 		}