For chroma coefficients that are masked (&= 0xfffff) to no value, the
context becomes in a weird state where it has no magnitude (ctx & 0x3f
== 0), but it does have a sign (ctx & 0xc0 != 0x40). Our old code
checked just the magnitude part of the context to set the skip context
of neighbouring blocks, but libaom uses both sign and magnitude for
this purpose. Therefore, adjust our code so it does the same thing.

Luma code only checks magnitude for this purpose and is thus not
affected by this peculiarity. Fixes #325.