Upgrade: Ensure unintelligible DB schemas don't result in content loss.

Merge of [32417] to the 4.1 branch.

See #32165.

Props ocean90.

Built from https://develop.svn.wordpress.org/branches/4.1@32419


git-svn-id: http://core.svn.wordpress.org/branches/4.1@32389 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/upgrade.php

@@ -1362,7 +1362,7 @@ function upgrade_415() {
 			);
 		}
 
-		if ( 'byte' !== $content_length['type'] ) {
+		if ( 'byte' !== $content_length['type'] || 0 === $content_length['length'] ) {
 			// Sites with malformed DB schemas are on their own.
 			return;
 		}