Fix coverity errors

[philljj]

Description

Fix miscellaneous things:

• check return value of mp_sub_d in wolfcrypt test.c
• check tmp != NULL before dereferencing in src ssl_asn1.c. tmp is assigned from buf, but then later buf pointer is potentially altered, making it unclear if tmp is guaranteed to != NULL (hence adding explicit check).
• fix potential double free or deref of freed encryptedContent in wolfcrypt pkcs7.c

Testing

Coverity scan.