Fix an off-by-one error in the sigset_t function error handling.

Fix an off-by-one error in the sigset_t function error handling.

Spotted while running the tests on MIPS, where sigset_t is
actually large enough. The bits in sigset_t are used such that
signal 1 is represented by bit 0, so the range of signals is
actually [1, 8*sizeof(sigset_t)]; it seems clearer to reword
the code in terms of valid bit offsets [0, 8*sizeof(sigset_t)),
which leads to the usual bounds checking idiom.

Change-Id: Id899c288e15ff71c85dd2fd33c47f8e97aa1956f