strbuf: make sure buffer is zero-terminated

strbuf: make sure buffer is zero-terminated

strbuf_init does not zero-terminate the initial buffer when hint is
non-zero. Fix this so we can rely on the string to be zero-terminated
even if we haven't filled it with anything yet.

Signed-off-by: Erik Faye-Lund <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>