Fix fd leak in pg_verifybackup

An error code path newly-introduced by 87ae9691 forgot to close a file
descriptor when verifying a file's checksum.

Per report from Coverity, via Tom Lane.

src/bin/pg_verifybackup/pg_verifybackup.c

@@ -730,6 +730,7 @@ verify_file_checksum(verifier_context *context, manifest_file *m,
 	{
 		report_backup_error(context, "could not initialize checksum of file \"%s\"",
 							relpath);
+		close(fd);
 		return;
 	}