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src/jit.rs: Fix offset when skipping insns for handling divisions by 0

When dividing by the content of a register, we have a specific case to
handle divisions by zero. When the denominator is not 0, we skip the
related instructions by emitted a near jump with the relevant offset.

However, this offset was incorrect. We accounted for two instructions: a
2-byte long XOR of the destination register with itself to set it to 0,
and a 5-byte long jump. The former may in fact emit 3 bytes under
certain condition, depending on the destination in use. For example,
when dividing from r7, we emit 3 bytes, and the fixed offset of 7 bytes
we used is incorrect, and triggers a segfault.

To fix this, let's add a check to determine the offset value we should
use. We also add related tests to make sure we don't regress on this in
the future.

Fixes: ce7d416d9a98 ("src/jit.rs: Update behaviour for divisions by zero (JIT)")
Fixes: https://github.com/qmonnet/rbpf/issues/88
Reported-by: Officeyutong
Signed-off-by: Quentin Monnet <quentin@isovalent.com>

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