Add assert for multipart boundary

Author: hnakamur

apache2/msc_multipart.c

@@ -993,6 +993,7 @@ int multipart_init(modsec_rec *msr, char **error_msg) {
  */
 int multipart_complete(modsec_rec *msr, char **error_msg) {
     assert(msr != NULL);
+    assert(msr->boundary != NULL);
     assert(error_msg != NULL);
     if (msr->mpd == NULL) return 1;