double64_init: Check psf->sf.channels against upper bound
This prevents division by zero later in the code.
While the trivial case to catch this (i.e. sf.channels < 1) has already
been covered, a crafted file may report a number of channels that is
so high (i.e. > INT_MAX/sizeof(double)) that it "somehow" gets
miscalculated to zero (if this makes sense) in the determination of the
blockwidth. Since we only support a limited number of channels anyway,
make sure to check here as well.
