Fix memory leak in mbedtls_md_setup with HMAC

mbedtls_md_setup() allocates a hash-specific context and then, if
requested, an extra HMAC context. If the second allocation failed, the
hash context was not freed.

Fix this by ensuring that the mbedtls_md_context_t object is always in
a consistent state, in particular, that the md_info field is always
set. For robustness, ensure that the object is in a consistent state
even on errors (other than BAD_INPUT_DATA if the object was not in a
consistent state on entry).

Fix #3486

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

ChangeLog.d/md_setup-leak.txt

@@ -0,0 +1,3 @@
+Bugfix
+   * Fix a memory leak in mbedtls_md_setup() when using HMAC under low memory
+     conditions. Reported and fix suggested by Guido Vranken in #3486.