worktree: fix leak in dwim_branch()

worktree: fix leak in dwim_branch()

Make sure that we release the temporary strbuf during dwim_branch() for
all codepaths (and not just for the early return).

This leak appears to have been introduced in:
  f60a7b763f (worktree: teach "add" to check out existing branches, 2018-04-24)

Note that UNLEAK(branchname) is still needed: the returned result is
used in add(), and is stored in a pointer which is used to point at one
of:
  - a string literal ("HEAD")
  - member of argv (whatever the user specified in their invocation)
  - or our newly allocated string returned from dwim_branch()
Fixing the branchname leak isn't impossible, but does not seem
worthwhile given that add() is called directly from cmd_main(), and
cmd_main() returns immediately thereafter - UNLEAK is good enough.

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

Direct leak of 60 byte(s) in 1 object(s) allocated from:
    #0 0x49a859 in realloc /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:164:3
    #1 0x9ab076 in xrealloc /home/ahunt/oss-fuzz/git/wrapper.c:126:8
    #2 0x939fcd in strbuf_grow /home/ahunt/oss-fuzz/git/strbuf.c:98:2
    #3 0x93af53 in strbuf_splice /home/ahunt/oss-fuzz/git/strbuf.c:239:3
    #4 0x83559a in strbuf_check_branch_ref /home/ahunt/oss-fuzz/git/object-name.c:1593:2
    #5 0x6988b9 in dwim_branch /home/ahunt/oss-fuzz/git/builtin/worktree.c:454:20
    #6 0x695f8f in add /home/ahunt/oss-fuzz/git/builtin/worktree.c:525:19
    #7 0x694a04 in cmd_worktree /home/ahunt/oss-fuzz/git/builtin/worktree.c:1036:10
    #8 0x4cd60d in run_builtin /home/ahunt/oss-fuzz/git/git.c:453:11
    #9 0x4cb2da in handle_builtin /home/ahunt/oss-fuzz/git/git.c:704:3
    #10 0x4ccc37 in run_argv /home/ahunt/oss-fuzz/git/git.c:771:4
    #11 0x4cac29 in cmd_main /home/ahunt/oss-fuzz/git/git.c:902:19
    #12 0x69caee in main /home/ahunt/oss-fuzz/git/common-main.c:52:11
    #13 0x7f7b7dd10349 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>