Fix error -Werror=sign-compare

Fix error -Werror=sign-compare

It blames comparing signed type with unsigned type.

Fix it in two step:
 1. If signed variable is definitely a nonnegative value at the
    code context, cast it into unsigned type directly.
 2. If it is unclear whether a signed variable is nonnegative, then
    add assertion for testing the variable is nonnegative. After
    that assertion, cast it into unsigned type. It helps catching
    violation that casting negative value into unsigned variable.

Change-Id: Ib4f2a2cb2ad1901518d0726de6035ac4a033bc67
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>