fast-import: check return value from unpack_entry()

fast-import: check return value from unpack_entry()

If the tree object we have asked for is deltafied in the packfile and
the delta did not apply correctly or was not able to be decompressed
from the packfile then we can get back NULL instead of the tree data.
This is (part of) the reason why read_sha1_file() can return NULL, so
we need to also handle it the same way.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>