fix undefined float behaviour in pack('f')

fix undefined float behaviour in pack('f')

The C standard says that the value of the expression (float)double_var is
undefined if 'the value being converted is outside the range of values
that can be represented'.

So to shut up -fsanitize=undefined:

    my $p = pack 'f', 1.36514538e67;

giving

    runtime error: value 1.36515e+67 is outside the range of representable values of type 'float'

explicitly handle the out of range values.
Something similar is already done under defined(VMS) && !defined(_IEEE_FP),
except that there it floors to +/- FLT_MAX rather than +/- (float)NV_INF.
I don't know which branch is best, and whether they should be merged.

This fix was suggested by Aaron Crane.