mtd: fix buffer leak and fd leak in mtd_dump()

Two leaks in mtd_dump():

- The buffer allocated with malloc(erasesize) is never freed before
  returning, leaking erasesize bytes on every call.
- The pre-existing malloc-NULL early return path also leaked the just-
  opened fd by returning directly instead of going through cleanup.

Initialize buf to NULL, route the malloc-NULL case through the
existing 'out:' label, and add free(buf) on the cleanup path so both
fd and buf are released consistently on every exit.

Signed-off-by: Anna Kiri <bredcorn@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23706
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>

package/system/mtd/src/mtd.c

@@ -367,7 +367,7 @@ mtd_dump(const char *mtd, int part_offset, int size)
 {
 	int ret = 0, offset = 0;
 	int fd;
-	char *buf;
+	char *buf = NULL;
 
 	if (quiet < 2)
 		fprintf(stderr, "Dumping %s ...\n", mtd);
@@ -385,8 +385,10 @@ mtd_dump(const char *mtd, int part_offset, int size)
 		lseek(fd, part_offset, SEEK_SET);
 
 	buf = malloc(erasesize);
-	if (!buf)
-		return -1;
+	if (!buf) {
+		ret = -1;
+		goto out;
+	}
 
 	do {
 		int len = (size > erasesize) ? (erasesize) : (size);
@@ -410,6 +412,7 @@ mtd_dump(const char *mtd, int part_offset, int size)
 	} while (size > 0);
 
 out:
+	free(buf);
 	close(fd);
 	return ret;
 }