sm: Improve the octet string cramming for pkcs#12

* sm/minip12.c (need_octet_string_cramming): New.
(tlv_expect_object, tlv_expect_octet_string): Run the test before
cramming.
* sm/minip12.c (ENABLE_DER_STRUCT_DUMPING): New but undefined macro
for debug purposes.
(bag_decrypted_data_p, bag_data_p): Use macro to allow dumping.

This bug was exhibited by importing a gpgsm exported EC certificate.

We use an extra test instead of retrying to allow retruning an error
from malloc failure. And well, for easier reading of the code.

(cherry picked from commit c1f78634ec3927ddcfdc4687bc6e408c658a0ece)

• GnuPG-bug-id: T6536