CCodec: fix underflow issue on handleImageData

the logic is assumed that gralloc does assume a contiguous mapping
at GraphicView2MediaImageConverter() in Codec2Buffer.
if it doesn't, underflow could happen because
type of variable is unsigned.

Bug: 168757280

Change-Id: I04e13d0680af74e76d96d3ab10a549f6368205cf
Signed-off-by: Taehwan Kim <t_h.kim@samsung.com>