mbedtls_pk_parse_key: don't allocate if not needed

mbedtls_pk_parse_key() makes a temporary copy of the key when it calls
pk_parse_key_pkcs8_encrypted_der(), because that function requires a
writable buffer. pk_parse_key_pkcs8_encrypted_der() always rejects an
empty password, so skip calling it in that case, which allows us to
skip the allocation as well.

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

ChangeLog.d/pkparse-pkcs8-unencrypted-no-alloc.txt

@@ -0,0 +1,3 @@
+Changes
+   * In mbedtls_pk_parse_key(), if no password is provided, don't allocate a
+     temporary variable on the heap. Suggested by Sergey Kanatov in #5304.

library/pkparse.c

@@ -1343,6 +1343,7 @@ int mbedtls_pk_parse_key( mbedtls_pk_context *pk,
      * error
      */
 #if defined(MBEDTLS_PKCS12_C) || defined(MBEDTLS_PKCS5_C)
+    if( pwdlen != 0 )
     {
         unsigned char *key_copy;