strbuf: avoid redundant reset in strbuf_getwholeline()

strbuf: avoid redundant reset in strbuf_getwholeline()

The HAVE_GETDELIM variant of strbuf_getwholeline() calls strbuf_reset()
on the strbuf before handing it over to getdelim(3).  This is
unnecessary:

  - getdelim(3) doesn't care whether the old buffer contents is
    NUL-terminated and has no access to ->len,
  - on success getdelim(3) NUL-terminates the buffer and we set ->len,
  - on error we either call strbuf_init() or strbuf_reset().

Remove the superfluous preparatory call.

Signed-off-by: René Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>