Add assertion for multipart boundary in multipart_complete

To satisfy Quality Gate in CI.
#3483 (comment)

Author: hnakamur

apache2/msc_multipart.c

@@ -1023,6 +1023,8 @@ int multipart_complete(modsec_rec *msr, char **error_msg) {
              * processed yet) in the buffer.
              */
             if (msr->mpd->buf_contains_line) {
+                assert(msr->mpd->boundary != NULL);
+
                 /*
                  * Note that the buffer may end with the final boundary followed by only CR,
                  * coming from the [CRLF epilogue], when allow_process_partial == 1 (which is