reset: free instead of leaking unneeded ref

dwim_ref() allocs a new string into ref. Instead of setting to NULL to
discard it, we can FREE_AND_NULL.

This leak appears to have been introduced in:
4cf76f6bbf (builtin/reset: compute checkout metadata for reset, 2020-03-16)

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

Direct leak of 5 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 0x9a7108 in xstrdup /home/ahunt/oss-fuzz/git/wrapper.c:29:14
    #2 0x8add6b in expand_ref /home/ahunt/oss-fuzz/git/refs.c:670:12
    #3 0x8ad777 in repo_dwim_ref /home/ahunt/oss-fuzz/git/refs.c:644:22
    #4 0x6394af in dwim_ref /home/ahunt/oss-fuzz/git/./refs.h:162:9
    #5 0x637e5c in cmd_reset /home/ahunt/oss-fuzz/git/builtin/reset.c:426:4
    #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 0x69c5ce in main /home/ahunt/oss-fuzz/git/common-main.c:52:11
    #11 0x7f57ebb9d349 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>

builtin/reset.c

@@ -425,7 +425,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 
 			dwim_ref(rev, strlen(rev), &dummy, &ref, 0);
 			if (ref && !starts_with(ref, "refs/"))
-				ref = NULL;
+				FREE_AND_NULL(ref);
 
 			err = reset_index(ref, &oid, reset_type, quiet);
 			if (reset_type == KEEP && !err)