Upgrade/Install: During the install process, add additional checking for exising tables.

This commit brings the changes in [49452] to the 5.2 branch.

If reinstalling WordPress, there is a condition where tables would exist in the database. Ensures that$

Fixes #51676.

Props xknown, garubi, mukesh27, desrosj, johnbillion, metalandcoffee, davidbaumwald, whyisjake.

Built from https://develop.svn.wordpress.org/branches/5.2@49456


git-svn-id: http://core.svn.wordpress.org/branches/5.2@49215 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -1567,7 +1567,10 @@ function is_blog_installed() {
 		}
 
 		$described_table = $wpdb->get_results( "DESCRIBE $table;" );
-		if ( is_array( $described_table ) && count( $described_table ) === 0 ) {
+		if (
+			( ! $described_table && empty( $wpdb->last_error ) ) ||
+			( is_array( $described_table ) && 0 === count( $described_table ) )
+		) {
 			continue;
 		}