bootutil: Fix memory leak in HKDF implementation

The bootutil_hmac_sha256_set_key routine performs some dynamic memory
allocations when mbedTLS is used. To properly free the allocated memory,
bootutil_hmac_sha256_drop must be called before reinitializing the HMAC
context using bootutil_hmac_sha256_init.  However, in the hkdf routine,
the HMAC context was freed only once even though it was initialized
multiple times.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>

boot/bootutil/src/encrypted.c

@@ -276,6 +276,8 @@ hkdf(uint8_t *ikm, uint16_t ikm_len, uint8_t *info, uint16_t info_len,
  goto error;
  }
 
+ bootutil_hmac_sha256_drop(&hmac);
+
  /*
  * Expand
  */
@@ -315,6 +317,8 @@ hkdf(uint8_t *ikm, uint16_t ikm_len, uint8_t *info, uint16_t info_len,
  goto error;
  }
 
+ bootutil_hmac_sha256_drop(&hmac);
+
  if (len > BOOTUTIL_CRYPTO_SHA256_DIGEST_SIZE) {
  memcpy(&okm[off], T, BOOTUTIL_CRYPTO_SHA256_DIGEST_SIZE);
  len -= BOOTUTIL_CRYPTO_SHA256_DIGEST_SIZE;
@@ -324,7 +328,6 @@ hkdf(uint8_t *ikm, uint16_t ikm_len, uint8_t *info, uint16_t info_len,
  }
  }
 
- bootutil_hmac_sha256_drop(&hmac);
  return 0;
 
 error: