compat: fix jrand48()

According to the manpage, jrand48() must return a signed long integer uniformly distributed over the interval [-2³¹, 2³¹).

The old implementation first computed iterate48(), which returns a positive 48-bit integer (as uint64_t, then cast to int64_t). Once right-shifted by 16 bits, the result is guaranteed to be a 32-bit positive integer as int64_t.

It is then (implicitly) cast to long to match the return type.

When the 32th bit is 0 (i.e. the value fits in 31-bit), then everything is ok. However, when the 32nd bit is 1, then there are two cases (which are both incorrect):

• if the long type is 32-bit on the platform, then conversion from int64_t to long is undefined;
• if the long type is more than 32-bit, then the resulting value will be a positive integer in the interval [0, 2³²), whereas jrand48() must return a value in the interval [-2³¹, 2³¹).