mbedtls: Update to version 2.16.12

This fixes the following security problems:
* Zeroize several intermediate variables used to calculate the expected
  value when verifying a MAC or AEAD tag. This hardens the library in
  case the value leaks through a memory disclosure vulnerability. For
  example, a memory disclosure vulnerability could have allowed a
  man-in-the-middle to inject fake ciphertext into a DTLS connection.
* Fix a double-free that happened after mbedtls_ssl_set_session() or
  mbedtls_ssl_get_session() failed with MBEDTLS_ERR_SSL_ALLOC_FAILED
  (out of memory). After that, calling mbedtls_ssl_session_free()
  and mbedtls_ssl_free() would cause an internal session buffer to
  be free()'d twice. CVE-2021-44732

The sizes of the ipk changed on MIPS 24Kc like this:
182454 libmbedtls12_2.16.11-2_mips_24kc.ipk
182742 libmbedtls12_2.16.12-1_mips_24kc.ipk

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 57f38e2c82)

package/libs/mbedtls/Makefile

@@ -8,13 +8,13 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=mbedtls
-PKG_VERSION:=2.16.11
+PKG_VERSION:=2.16.12
 PKG_RELEASE:=1
 PKG_USE_MIPS16:=0
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=https://codeload.github.com/ARMmbed/mbedtls/tar.gz/v$(PKG_VERSION)?
-PKG_HASH:=c18e7e9abf95e69e425260493720470021384a1728417042060a35d0b7b18b41
+PKG_HASH:=294871ab1864a65d0b74325e9219d5bcd6e91c34a3c59270c357bb9ae4d5c393
 
 PKG_BUILD_PARALLEL:=1
 PKG_LICENSE:=GPL-2.0+