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copy_offset_int_big was assuming (Offset + Size - 1) (Tmp9 in the first
BB) would not underflow. It was also unconditionally reading and writing
the binary even when Size was zero, unlike copy_int_little, which is the
only other case of bs_put_integer that does not have a short-circuit on
Size = 0.
This was causing segfaults when constructing binaries starting with a
zero-length integer field, because a logical right shift was used to
compute an offset in bytes (which became 0x1fffffffffffffff) to read in
the binary.
Tests, taken from the emulator bs_construct_SUITE, were also added.
The complete credit for the report and the fix goes to Magnus Lång.
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