Add sanity checks before allocations

[ecton]

Many times when reading data from disk, we read a length and immediately ask the OS to ensure a Vec can hold that much data, potentially resizing the Vec.

If there is an issue with the data on-disk, this could lead to attempting to allocate too much data. The subsequent read call will fail due to an EOF, if the allocation succeeds.

Before requesting a resize or reservation, we should check that the bytes requested are less than or equal to the number of bytes that remain in the reader.