open_istream(): do not dereference NULL in the error case

open_istream(): do not dereference NULL in the error case

When stream-filter cannot be attached, it is expected to return NULL,
and we should close the stream we opened and signal an error by
returning NULL ourselves from this function.

However, we attempted to dereference that NULL pointer between the
point we detected the error and returned from the function.

Brought-to-attention-by: John Keeping <redacted>
Signed-off-by: Junio C Hamano <redacted>