builtin/merge: free found_ref when done

merge_name() calls dwim_ref(), which allocates a new string into
found_ref. Therefore add a free() to avoid leaking found_ref.

LSAN output from t0021:

Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x486804 in strdup ../projects/compiler-rt/lib/asan/asan_interceptors.cpp:452:3
    #1 0xa8beb8 in xstrdup wrapper.c:29:14
    #2 0x954054 in expand_ref refs.c:671:12
    #3 0x953cb6 in repo_dwim_ref refs.c:644:22
    #4 0x5d3759 in dwim_ref refs.h:162:9
    #5 0x5d3759 in merge_name builtin/merge.c:517:6
    #6 0x5d3759 in collect_parents builtin/merge.c:1214:5
    #7 0x5cf60d in cmd_merge builtin/merge.c:1458:16
    git#8 0x4ce83e in run_builtin git.c:475:11
    git#9 0x4ccafe in handle_builtin git.c:729:3
    git#10 0x4cb01c in run_argv git.c:818:4
    git#11 0x4cb01c in cmd_main git.c:949:19
    git#12 0x6bdbfd in main common-main.c:52:11
    git#13 0x7f0430502349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 16 byte(s) leaked in 1 allocation(s).

Signed-off-by: Andrzej Hunt <andrzej@ahunt.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -503,7 +503,7 @@ static void merge_name(const char *remote, struct strbuf *msg)
 	struct strbuf bname = STRBUF_INIT;
 	struct merge_remote_desc *desc;
 	const char *ptr;
-	char *found_ref;
+	char *found_ref = NULL;
 	int len, early;
 
 	strbuf_branchname(&bname, remote, 0);
@@ -586,6 +586,7 @@ static void merge_name(const char *remote, struct strbuf *msg)
 	strbuf_addf(msg, "%s\t\tcommit '%s'\n",
 		oid_to_hex(&remote_head->object.oid), remote);
 cleanup:
+	free(found_ref);
 	strbuf_release(&buf);
 	strbuf_release(&bname);
 }