test: can we create 0-length cstructs (yes, we can)
test: check that shift works in the +ve direction
test: check that it's illegal to shift beyond the buffer length

This test fails, the resulting cstruct will have a -ve length
test: check that we can't shift <-- before the start of the buffer
test: check (Cstruct.create (-1)) fails and (Cstruct.sub works)
test: check that 'sub' fails if the length is too big
test: check that 'sub' fails if the length is too small (ie -ve)
test: sub with too large an offset fails
test: check that 'sub' can't make the offset go negative
test: set_len fails if the length is too big
test: check set_len fails for -ve lengths
test: check add_len fails when len is too big
test: check add_len fails with -ve lengths
test: blit with too large a source offset fails
test: check blit fails if the offset is -ve
test: blit dest offset, just like the source offset
test: check blit fails if the length is too big
test: blit fails when the length is -ve
Check for -ve offsets and lengths. This fixes the broken unit tests.
Update CHANGES

Signed-off-by: David Scott <dave.scott@eu.citrix.com>