strbuf_setlen(): do not barf on setting length of an empty buffer to 0

strbuf_setlen() expect to be able to NUL terminate the buffer,
but a completely empty strbuf could have an empty buffer with 0
allocation; both the assert() and the assignment for NUL
termination would fail.

Signed-off-by: Junio C Hamano <gitster@pobox.com>