Fix overflow bug in ldexp(x, exp). The 'exp' argument maps to a C int for the
math library call [double ldexp(double, int)], however the 'd'
PyArg_ParseTuple formatter was used to yield a double, which was subsequently
cast to an int. This could overflow.

[GvR: mysteriously, on Solaris 2.7, ldexp(1, 2147483647) returns Inf
while ldexp(1, 2147483646) raises OverflowError; this seems a bug in
the math library (it also takes a real long time to compute the
Inf outcome).  Does this point to a bug in the CHECK() macro?  It
should have discovered that the result was outside the HUGE_VAL range.]