bugfix: fix "attempt to shift left with overflow" exception

[mwicat]

I spotted this because it causes a few tests to fail.

The expression inside decode_truncated_block_i32 function that is used to get a bitmask for given bit_size will fail in case bit_size == 32. Expression 1<<32 will be evaluated first which will cause "attempt to shift left with overflow" exception since 1<<32 value does not fit in i32 type. Let's compute the bitmask incrementally from zero instead of backward arithmetic to avoid the type overflow.

[isolin]

The same should be then used in read_packed_values_i32 as well. In fact, it would be the best to have a small function just doing (in pseudocode):

x == 32 ? -1 : (x == 31 ? 2147483647 : (1 << x) - 1)

If I counted well, it will be called 2x (incl. your previous patch).

Then, there is another call on the next line, which also needs handling for bit_size = 32:

x == 32 ? -2147483648 : 1 << (x - 1))