Found with libfuzzer. The length is automatically converted to int (thus
large sizes could become negative, even though they are legal "array sizes").
Besides that, the length is coerced to a SMI (which is an even tighter
constraint on 32-bit systems, where it limits the legal sizes to 2^30 - 1).

Add checks that the length of a dense array is below that threshold, and also
fail fast if a length that is provided obviously could not be the correct dense
length (because there isn't enough data left in the buffer to populate such an
array).

BUG=chromium:148757

Review-Url: https://codereview.chromium.org/2399873002
Cr-Commit-Position: refs/heads/master@{#40094}