[CritFix] mime_parser: avoid NULL deref on SMIME with empty pkcs7-data

[CritFix] mime_parser: avoid NULL deref on SMIME with empty pkcs7-data

When an S/MIME signed message wraps an inner pkcs7-data with a zero-length
OCTET STRING, the SMIME inner-content extraction in rspamd_mime_parse_normal_part
allocated a zero-length buffer and recursed into rspamd_mime_process_multipart_node
with start/end pointing at NULL (g_malloc(0) returns NULL under always_malloc
mempool mode), causing a SIGSEGV at the first byte check.

Fix:
- Skip the SMIME inner recursion when the encapsulated OCTET STRING is empty
  or has a NULL data pointer.
- Add a defensive guard at the top of rspamd_mime_process_multipart_node to
  return RSPAMD_MIME_PARSE_NO_PART for NULL or empty buffers, protecting any
  other caller from the same UB.

Add a Lua regression test that exercises the SMIME-empty path through
rspamd_message_parse. With VALGRIND=1 (forcing always_malloc) the test
reliably reproduced the crash before the fix.