attr: fix integer overflow with more than INT_MAX macros

attr: fix integer overflow with more than INT_MAX macros

Attributes have a field that tracks the position in the `all_attrs`
array they're stored inside. This field gets set via `hashmap_get_size`
when adding the attribute to the global map of attributes. But while the
field is of type `int`, the value returned by `hashmap_get_size` is an
`unsigned int`. It can thus happen that the value overflows, where we
would now dereference teh `all_attrs` array at an out-of-bounds value.

We do have a sanity check for this overflow via an assert that verifies
the index matches the new hashmap's size. But asserts are not a proper
mechanism to detect against any such overflows as they may not in fact
be compiled into production code.

Fix this by using an `unsigned int` to track the index and convert the
assert to a call `die()`.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>