symbolic-ref: don't leak shortened refname in check_symref()

symbolic-ref: don't leak shortened refname in check_symref()

shorten_unambiguous_ref() returns an allocated string. We have to
track it separately from the const refname.

This leak has existed since:
9ab55daa55 (git symbolic-ref --delete $symref, 2012-10-21)

This leak was found when running t0001 with LSAN, see also LSAN output
below:

Direct leak of 19 byte(s) in 1 object(s) allocated from:
    #0 0x486514 in strdup /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/asan/asan_interceptors.cpp:452:3
    #1 0x9ab048 in xstrdup /home/ahunt/oss-fuzz/git/wrapper.c:29:14
    #2 0x8b452f in refs_shorten_unambiguous_ref /home/ahunt/oss-fuzz/git/refs.c
    #3 0x8b47e8 in shorten_unambiguous_ref /home/ahunt/oss-fuzz/git/refs.c:1287:9
    #4 0x679fce in check_symref /home/ahunt/oss-fuzz/git/builtin/symbolic-ref.c:28:14
    #5 0x679ad8 in cmd_symbolic_ref /home/ahunt/oss-fuzz/git/builtin/symbolic-ref.c:70:9
    #6 0x4cd60d in run_builtin /home/ahunt/oss-fuzz/git/git.c:453:11
    #7 0x4cb2da in handle_builtin /home/ahunt/oss-fuzz/git/git.c:704:3
    #8 0x4ccc37 in run_argv /home/ahunt/oss-fuzz/git/git.c:771:4
    #9 0x4cac29 in cmd_main /home/ahunt/oss-fuzz/git/git.c:902:19
    #10 0x69cc6e in main /home/ahunt/oss-fuzz/git/common-main.c:52:11
    #11 0x7f98388a4349 in __libc_start_main (/lib64/libc.so.6+0x24349)

Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>