Add assertion for multipart boundary in multipart_complete

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

Author: hnakamur

apache2/msc_multipart.c

@@ -1023,10 +1023,7 @@ int multipart_complete(modsec_rec *msr, char **error_msg) {
              * processed yet) in the buffer.
              */
             if (msr->mpd->buf_contains_line) {
-                if (msr->mpd->boundary == NULL) {
-                    *error_msg = apr_psprintf(msr->mp, "Multipart: BUG: multipart_complete must not be called if multipart_init returns an error.");
-                    return -1;
-                }
+                assert(msr->mpd->boundary != NULL);
 
                 /*
                  * Note that the buffer may end with the final boundary followed by only CR,