unpack.c: Cast negative signed offset to signedHEADmaster

As N1570§6.5.16.2p3 states, E1 += E2 is equivalent to E1 = E1 + (E2).
This means that E1 + (E2) will be subject to (6.5.6p4) "the usual
arithmetic conversions". Since both types have equal integer conversion
rank (6.3.1.1p1), E1 which was intmax_t will first be converted to
uintmax_t before being added to (E2). This is not the intended result as
it relies on integer wrapping behaviour again. Casting E2 to intmax_t
first prevents the conversion of E1 and produces the correct result.

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