branch: FREE_AND_NULL instead of NULL'ing real_ref

branch: FREE_AND_NULL instead of NULL'ing real_ref

real_ref was previously populated by dwim_ref(), which allocates new
memory. We need to make sure to free real_ref when discarding it.
(real_ref is already being freed at the end of create_branch() - but
if we discard it early then it will leak.)

This fixes the following leak found while running t0002-t0099:

Direct leak of 5 byte(s) in 1 object(s) allocated from:
    #0 0x486954 in strdup /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/asan/asan_interceptors.cpp:452:3
    #1 0xdd6484 in xstrdup wrapper.c:29:14
    #2 0xc0f658 in expand_ref refs.c:671:12
    #3 0xc0ecf1 in repo_dwim_ref refs.c:644:22
    #4 0x8b1184 in dwim_ref ./refs.h:162:9
    #5 0x8b0b02 in create_branch branch.c:284:10
    #6 0x550cbb in update_refs_for_switch builtin/checkout.c:1046:4
    #7 0x54e275 in switch_branches builtin/checkout.c:1274:2
    #8 0x548828 in checkout_branch builtin/checkout.c:1668:9
    #9 0x541306 in checkout_main builtin/checkout.c:2025:9
    #10 0x5395fa in cmd_checkout builtin/checkout.c:2077:8
    #11 0x4d02a8 in run_builtin git.c:467:11
    #12 0x4cbfe9 in handle_builtin git.c:719:3
    #13 0x4cf04f in run_argv git.c:808:4
    #14 0x4cb85a in cmd_main git.c:939:19
    #15 0x820cf6 in main common-main.c:52:11
    #16 0x7f30bd9dd349 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>