khash: clarify that allocations never fail

khash: clarify that allocations never fail

We use our standard allocation functions and macros (xcalloc,
ALLOC_ARRAY, REALLOC_ARRAY) in our version of khash.h.  They terminate
the program on error instead, so code that's using them doesn't have to
handle allocation failures.  Make this behavior explicit by turning
kh_resize_ into a void function and removing the related unreachable
error handling code.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>