Fix the implementation of StreamingXXHash32.asChecksum()

[trazfr]

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In the Checksum view, we only keep the last 28 bits of the hash instead of the 32 bits of the full hash.

The result is that the 4 first bits are always set to 0 in LZ4Block*Stream. This PR:

• fixes the Checksum view in StreamingXXHash32
• keeps the current behaviour in LZ4Block*Stream to avoid breaking existing data built with this library