libsparse: Fix overflow of merged sparse chunk length

Merging sparse chunk can make sparse map block bigger than 4GiB,
that can't be covered by unsigned integer type. Fix this by
changing unsigned int to uint64_t type.

Test: sparse build
Bug: 162808120
Change-Id: Id4d3f88f9d531c25c3937c99b2c81efb915605ee
Signed-off-by: Hyeongseok Kim <hyeongseok@gmail.com>
Cc: hyeongseok.kim <hyeongseok.kim@lge.com>