fill_textconv(): Don't get/put cache if sha1 is not valid

fill_textconv(): Don't get/put cache if sha1 is not valid

When blaming files in the working tree, the filespec is marked with
!sha1_valid, as we have not given the contents an object name yet.  The
function to cache textconv results (keyed on the object name), however,
didn't check this condition, and ended up on storing the cached result
under a random object name.

Cc: Axel Bonnet <axel.bonnet@ensimag.imag.fr>
Cc: Clément Poulain <clement.poulain@ensimag.imag.fr>
Cc: Diane Gasselin <diane.gasselin@ensimag.imag.fr>
Cc: Jeff King <peff@peff.net>
Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>