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

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
    #8 0x410dcd in handle_builtin git.c:729
    #9 0x414087 in run_argv git.c:818
    #10 0x414087 in cmd_main git.c:949
    #11 0x40e9ec in main common-main.c:52
    #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>