packfile.c: prevent overflow in `load_idx()`

packfile.c: prevent overflow in `load_idx()`

Prevent an overflow when locating a pack's CRC offset when the number
of packed items is greater than 2^32-1/hashsz by guarding the
computation with an `st_mult()`.

Note that to avoid truncating the result, the `crc_offset` member must
itself become a `size_t`. The only usage of this variable (besides the
assignment in `load_idx()`) is in `read_v2_anomalous_offsets()` in the
index-pack code. There we use the `crc_offset` as a pointer offset, so
we are already equipped to handle the type change.

Helped-by: Phillip Wood <redacted>
Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>