smtp: skip preprocessing for mime headers

Mime parser doesn't expect partial lines, which preprocessing can
provide. Add a check to let mime headers be handled by regular line
parsing.

src/app-layer-smtp.c

@@ -1275,6 +1275,11 @@ static int SMTPPreProcessCommands(
 {
     DEBUG_VALIDATE_BUG_ON((state->parser_state & SMTP_PARSER_STATE_COMMAND_DATA_MODE) == 0);
 
+    /* fall back to strict line parsing for mime header parsing */
+    if (state->curr_tx && state->curr_tx->mime_state &&
+            state->curr_tx->mime_state->state_flag < HEADER_DONE)
+        return 1;
+
     bool line_complete = false;
     const int32_t input_len = input->len;
     for (int32_t i = 0; i < input_len; i++) {

src/util-decode-mime.c

@@ -1706,6 +1706,9 @@ static int FindMimeHeader(const uint8_t *buf, uint32_t blen,
     int finish_header = 0, new_header = 0;
     MimeDecConfig *mdcfg = MimeDecGetConfig();
 
+    /* should not get here with incomplete lines */
+    DEBUG_VALIDATE_BUG_ON(state->current_line_delimiter_len == 0);
+
     /* Find first header */
     hname = FindMimeHeaderStart(buf, blen, &hlen);
     if (hname != NULL) {