smtp: fix null deref with config option body md5

Ticket: #6279

If we have the smtp body beginning without headers, we need to
create the md5 context and right away and supply data to it.
Otherwise, on the next line being processed, md5_ctx will be
NULL but body_begin will have been reset to 0

src/util-decode-mime.c

@@ -1766,6 +1766,12 @@ static int FindMimeHeader(const uint8_t *buf, uint32_t blen,
         state->body_begin = 1;
         state->body_end = 0;
 
+        // Begin the body md5 computation if config asks so
+        if (MimeDecGetConfig()->body_md5 && state->md5_ctx == NULL) {
+            state->md5_ctx = SCMd5New();
+            SCMd5Update(state->md5_ctx, buf, blen + state->current_line_delimiter_len);
+        }
+
         ret = ProcessBodyLine(buf, blen, state);
         if (ret != MIME_DEC_OK) {
             SCLogDebug("Error: ProcessBodyLine() function failed");