out-of-bounds read in crc32sseAll

[klauspost]

From klauspost/crc32#4

There is a 1 byte out-of bound read on (length & 7) == 7.

[klauspost]

When length was 7, an extra byte would be read from the input. In rare cases that would hit a page boundary when flushing and crash.

The test has been extended to increase the chance of detecting a situation like this.