builtin/submodule--helper: release unused strbuf to avoid leak

relative_url() populates sb. In the normal return path, its buffer is
detached using strbuf_detach(). However the early return path does
nothing with sb, which means that sb's memory is leaked - therefore
we add a release to avoid this leak.

The reset is also only necessary for the normal return path, hence we
move it down to after the early-return to avoid unnecessary work.

LSAN output from t0060:

Direct leak of 121 byte(s) in 1 object(s) allocated from:
    #0 0x7f31246f28b0 in realloc (/usr/lib64/libasan.so.4+0xdc8b0)
    #1 0x98d7d6 in xrealloc wrapper.c:126
    #2 0x909a60 in strbuf_grow strbuf.c:98
    #3 0x90bf00 in strbuf_vaddf strbuf.c:401
    #4 0x90c321 in strbuf_addf strbuf.c:335
    #5 0x5cb78d in relative_url builtin/submodule--helper.c:182
    #6 0x5cbe46 in resolve_relative_url_test builtin/submodule--helper.c:248
    #7 0x410dcd in run_builtin git.c:475
    git#8 0x410dcd in handle_builtin git.c:729
    git#9 0x414087 in run_argv git.c:818
    git#10 0x414087 in cmd_main git.c:949
    git#11 0x40e9ec in main common-main.c:52
    git#12 0x7f3123c41349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 121 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/submodule--helper.c

@@ -187,11 +187,13 @@ static char *relative_url(const char *remote_url,
 		out = xstrdup(sb.buf + 2);
 	else
 		out = xstrdup(sb.buf);
-	strbuf_reset(&sb);
 
-	if (!up_path || !is_relative)
+	if (!up_path || !is_relative) {
+		strbuf_release(&sb);
 		return out;
+	}
 
+	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s%s", up_path, out);
 	free(out);
 	return strbuf_detach(&sb, NULL);