uclient-fetch: fix potential memory leaks

uclient-fetch: fix potential memory leaks

Fixes following issue reported by clang-12 static analyzer:

 uclient-fetch.c:612:25: warning: Potential leak of memory pointed to by 'username' [unix.Malloc]
                                 memset(optarg, '*', strlen(optarg));
                                                     ^~~~~~~~~~~~~~
 uclient-fetch.c:618:25: warning: Potential leak of memory pointed to by 'password' [unix.Malloc]
                                 memset(optarg, '*', strlen(optarg));
                                                     ^~~~~~~~~~~~~~

Signed-off-by: Petr Štetiar <ynezz@true.cz>