mtd: parsers: Fix memory leak in mtd_parser_tplink_safeloader_parse()

The function mtd_parser_tplink_safeloader_parse() allocates buf via
mtd_parser_tplink_safeloader_read_table(). If the allocation for
parts[idx].name fails inside the loop, the code jumps to the err_free
label without freeing buf, leading to a memory leak.

Fix this by freeing the temporary buffer buf in the err_free label.

Compile tested only. Issue found using a prototype static analysis tool
and code review.

Fixes: 00a3588084be ("mtd: parsers: add TP-Link SafeLoader partitions table parser")
Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>