* alloc.c: Check that resized vectors' lengths fit in fixnums.

* alloc.c: Check that resized vectors' lengths fit in fixnums.

(header_size, word_size): New constants.
(allocate_vectorlike): Don't check size overflow here.
(allocate_vector): Check it here instead, since this is the only
caller of allocate_vectorlike that could cause overflow.
Check that the new vector's length is representable as a fixnum.