PKCS#7: Provide a single place to do signed info block freeing

The code to free a signed info block is repeated several times, so move the
code to do it into a function of its own.  This gives us a place to add clean
ups for stuff that gets added to pkcs7_signed_info.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>