dec/vp8l.c: add assertions in EmitRescaledRowsRGBA/YUVA
dec/vp8l.c: add assertions in EmitRescaledRowsRGBA/YUVA

Add assertions to make sure the number of lines that are imported
into rescaler is as expected.

Change-Id: I3e9e4202e696bb09f7aa54519285b6b0deb84c1a

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp