ftp: only consider entry path if it has a length

Follow-up from 8edcfedc1a

Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=65631

Avoids a NULL pointer deref.

Closes #12648

lib/ftp.c

@@ -2892,6 +2892,7 @@ static CURLcode ftp_statemachine(struct Curl_easy *data,
               }
               else {
                 /* end of path */
+                if(Curl_dyn_len(&out))
                   entry_extracted = TRUE;
                 break; /* get out of this loop */
               }