add asserts on Sk4px::Map functions that our arrays are non-null.
add asserts on Sk4px::Map functions that our arrays are non-null.

There's a crasher (null dereference it looks like) in Clank with an impossible stack, but with blit mask on top (apparently sk_free() calls blit mask...).  Can't hurt to check that all these Sk4px-using methods are iterating over non-null arrays.

(This is that bug I mentioned that got me thinking about blit mask in the first place.)

BUG=520354

Review URL: https://codereview.chromium.org/1295443002

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